Symmetric motion vector difference coding

ABSTRACT

Bi-directional optical flow (BDOF) may be bypassed, for a current coding block, based on whether symmetric motion vector difference (8MVD) is used in motion vector coding for the current coding block, A coding device (e.g., an encoder or a decoder) may determine whether to bypass BDOF for the current coding block based at least in part on an SMVD indication for the current coding block, The coding device may obtain the SMVD indication that indicates whether SMVD is used in motion vector coding for the current coding block. If SMVD Indication indicates that SMVD is used in the motion vector coding for the current coding block, the coding device may bypass BDOF for the current coding block. The coding device may reconstruct, the current coding block without performing BDOF if it determines to bypass BDOF for the current coding block.

CROSS-REFERENCE TO RELATED CASES

The present application is the National Stage Entry under 35 U.S.C. §371 of Patent Cooperation Treaty Application No. PCT/US2019/067527,filed Dec. 19, 2019, which claims the benefit of U.S. Provisional PatentApplication No. 62/783,437, filed Dec. 21, 2018, U.S. Provisional PatentApplication No. 62/787,321, filed Jan. 1, 2019, U.S. Provisional PatentApplication No. 62/792,710, filed Jan. 15, 2019, U.S. Provisional PatentApplication No. 62/798,674, filed Jan. 30, 2019, and U.S. ProvisionalPatent Application No. 62/809,308, filed Feb. 22, 2019, the contents ofwhich are hereby incorporated by reference in their entireties.

BACKGROUND

Video coding systems may be used to compress digital video signals,e.g., to reduce the storage and/or transmission bandwidth needed forsuch signals. Video coding systems may include block-based,wavelet-based, and/or object-based systems. The systems employ videocoding techniques, such as bi-directional motion compensated prediction(MCP), which may remove temporal redundancies by exploiting temporalcorrelations between pictures. Such techniques may increase thecomplexity of computations performed during encoding and/or decoding.

SUMMARY

Bi-directional optical flow (BDOF) may be bypassed, for a current codingblock, based on whether symmetric motion vector difference (SMVD) isused in motion vector coding for the current coding block.

A coding device (e.g., an encoder or a decoder) may determine that BDOFis enabled. The coding device may determine whether to bypass BDOF forthe current coding block based at least in part on an SMVD indicationfor the current coding block. The coding device may obtain the SMVDindication that indicates whether SMVD is used in motion vector codingfor the current coding block. If SMVD indication indicates that SMVD isused in the motion vector coding for the current coding block, thecoding device may bypass BDOF for the current coding block. The codingdevice may reconstruct the current coding block without performing BDOFif it determines to bypass BDOF for the current coding block.

A motion vector difference (MVD) for the current coding block mayindicate a difference between a motion vector predictor (MVP) for thecurrent coding block and a motion vector (MV) for the current codingblock. The MVP for the current coding block may be determined based onan MV of a spatial neighboring block of the current coding block and/ora temporal neighboring block of the current coding block.

If an SMVD indication indicates SMVD is used for the motion vectorcoding for the current coding block, the coding device may receive afirst motion vector coding information associated with a first referencepicture list. The coding device may determine a second motion vectorcoding information associated with a second reference picture list basedon the first motion vector coding information associated with the firstreference picture list and that the MVD associated with the firstreference picture list and the MVD associated with the second referencepicture list are symmetric.

In an example, if the SMVD indication indicates SMVD is used for themotion vector coding for the current coding block, the coding device mayparse a first MVD associated with a first reference picture list in abitstream. The coding device may determine a second MVD associated witha second reference picture list based on the first MVD and that thefirst MVD and the second MVD are symmetric to each other.

If the coding device determines not to bypass BDOF for the currentcoding block, the coding device may refine a motion vector of a (e.g.,each) sub-block of the current coding block based at least in part ongradients associated with a location in the current coding block.

The coding device may receive a sequence-level SMVD indication thatindicates whether SMVD is enabled for a sequence of pictures. If SMVD isenabled for the sequence of pictures, the coding device may obtain theSMVD indication associated with the current coding block based on thesequence-level SMVD indication.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a system diagram illustrating an example communicationssystem in which one or more disclosed embodiments may be implemented.

FIG. 1B is a system diagram illustrating an example wirelesstransmit/receive unit (WTRU) that may be used within the communicationssystem illustrated in FIG. 1A according to an embodiment.

FIG. 1C is a system diagram illustrating an example radio access network(RAN) and an example core network (CN) that may be used within thecommunications system illustrated in FIG. 1A according to an embodiment.

FIG. 1D is a system diagram illustrating a further example RAN and afurther example CN that may be used within the communications systemillustrated in FIG. 1A according to an embodiment.

FIG. 2 is a diagram of an example block-based hybrid video encodingframework for an encoder.

FIG. 3 is a diagram of an example block-based video decoding frameworkfor a decoder.

FIG. 4 is a diagram of an example video encoder with support ofbi-prediction with CU weights (e.g., GBi).

FIG. 5 is a diagram of an example module with support for bi-predictionwith CU weights for an encoder.

FIG. 6 is a diagram of an example block-based video decoder with supportfor bi-prediction with CU weights.

FIG. 7 is a diagram of an example module with support for bi-predictionwith CU weights for a decoder.

FIG. 8 illustrates an example bidirectional optical flow.

FIG. 9 illustrates an example four-parameter affine mode.

FIG. 10 illustrates an example six-parameter affine mode.

FIG. 11 illustrates an example non-affine motion symmetric MVD (e.g.,MVD1=−MVD0).

FIG. 12 illustrates an example motion vector difference (MVD) searchpoint(s) (e.g., for merge mode MVD).

FIG. 13 illustrates an example affine motion symmetric MVD.

DETAILED DESCRIPTION

A more detailed understanding may be had from the following description,given by way of example in conjunction with the accompanying drawings.

FIG. 1A is a diagram illustrating an example communications system 100in which one or more disclosed embodiments may be implemented. Thecommunications system 100 may be a multiple access system that providescontent, such as voice, data, video, messaging, broadcast, etc., tomultiple wireless users. The communications system 100 may enablemultiple wireless users to access such content through the sharing ofsystem resources, including wireless bandwidth. For example, thecommunications systems 100 may employ one or more channel accessmethods, such as code division multiple access (CDMA), time divisionmultiple access (TDMA), frequency division multiple access (FDMA),orthogonal FDMA (OFDMA), single-carrier FDMA (SC-FDMA), zero-tailunique-word DFT-Spread OFDM (ZT UW DTS-s OFDM), unique word OFDM(UW-OFDM), resource block-filtered OFDM, filter bank multicarrier(FBMC), and the like.

As shown in FIG. 1A, the communications system 100 may include wirelesstransmit/receive units (WTRUs) 102 a, 102 b, 102 c, 102 d, a RAN104/113, a CN 106/115, a public switched telephone network (PSTN) 108,the Internet 110, and other networks 112, though it will be appreciatedthat the disclosed embodiments contemplate any number of WTRUs, basestations, networks, and/or network elements. Each of the WTRUs 102 a,102 b, 102 c, 102 d may be any type of device configured to operateand/or communicate in a wireless environment. By way of example, theWTRUs 102 a, 102 b, 102 c, 102 d, any of which may be referred to as a“station” and/or a “STA”, may be configured to transmit and/or receivewireless signals and may include a user equipment (UE), a mobilestation, a fixed or mobile subscriber unit, a subscription-based unit, apager, a cellular telephone, a personal digital assistant (PDA), asmartphone, a laptop, a netbook, a personal computer, a wireless sensor,a hotspot or Mi-Fi device, an Internet of Things (IoT) device, a watchor other wearable, a head-mounted display (HMD), a vehicle, a drone, amedical device and applications (e.g., remote surgery), an industrialdevice and applications (e.g., a robot and/or other wireless devicesoperating in an industrial and/or an automated processing chaincontexts), a consumer electronics device, a device operating oncommercial and/or industrial wireless networks, and the like. Any of theWTRUs 102 a, 102 b, 102 c and 102 d may be interchangeably referred toas a UE. The coding device may be or may include a WTRU.

The communications systems 100 may also include a base station 114 aand/or a base station 114 b. Each of the base stations 114 a, 114 b maybe any type of device configured to wirelessly interface with at leastone of the WTRUs 102 a, 102 b, 102 c, 102 d to facilitate access to oneor more communication networks, such as the CN 106/115, the Internet110, and/or the other networks 112. By way of example, the base stations114 a, 114 b may be a base transceiver station (BTS), a Node-B, an eNodeB, a Home Node B, a Home eNode B, a gNB, a NR NodeB, a site controller,an access point (AP), a wireless router, and the like. While the basestations 114 a, 114 b are each depicted as a single element, it will beappreciated that the base stations 114 a, 114 b may include any numberof interconnected base stations and/or network elements.

The base station 114 a may be part of the RAN 104/113, which may alsoinclude other base stations and/or network elements (not shown), such asa base station controller (BSC), a radio network controller (RNC), relaynodes, etc. The base station 114 a and/or the base station 114 b may beconfigured to transmit and/or receive wireless signals on one or morecarrier frequencies, which may be referred to as a cell (not shown).These frequencies may be in licensed spectrum, unlicensed spectrum, or acombination of licensed and unlicensed spectrum. A cell may providecoverage for a wireless service to a specific geographical area that maybe relatively fixed or that may change over time. The cell may furtherbe divided into cell sectors. For example, the cell associated with thebase station 114 a may be divided into three sectors. Thus, in oneembodiment, the base station 114 a may include three transceivers, i.e.,one for each sector of the cell. In an embodiment, the base station 114a may employ multiple-input multiple output (MIMO) technology and mayutilize multiple transceivers for each sector of the cell. For example,beamforming may be used to transmit and/or receive signals in desiredspatial directions.

The base stations 114 a, 114 b may communicate with one or more of theWTRUs 102 a, 102 b, 102 c, 102 d over an air interface 116, which may beany suitable wireless communication link (e.g., radio frequency (RF),microwave, centimeter wave, micrometer wave, infrared (IR), ultraviolet(UV), visible light, etc.). The air interface 116 may be establishedusing any suitable radio access technology (RAT).

More specifically, as noted above, the communications system 100 may bea multiple access system and may employ one or more channel accessschemes, such as CDMA, TDMA, FDMA, OFDMA, SC-FDMA, and the like. Forexample, the base station 114 a in the RAN 104/113 and the WTRUs 102 a,102 b, 102 c may implement a radio technology such as Universal MobileTelecommunications System (UMTS) Terrestrial Radio Access (UTRA), whichmay establish the air interface 115/116/117 using wideband CDMA (WCDMA).WCDMA may include communication protocols such as High-Speed PacketAccess (HSPA) and/or Evolved HSPA (HSPA+). HSPA may include High-SpeedDownlink (DL) Packet Access (HSDPA) and/or High-Speed UL Packet Access(HSUPA).

In an embodiment, the base station 114 a and the WTRUs 102 a, 102 b, 102c may implement a radio technology such as Evolved UMTS TerrestrialRadio Access (E-UTRA), which may establish the air interface 116 usingLong Term Evolution (LTE) and/or LTE-Advanced (LTE-A) and/orLTE-Advanced Pro (LTE-A Pro).

In an embodiment, the base station 114 a and the WTRUs 102 a, 102 b, 102c may implement a radio technology such as NR Radio Access, which mayestablish the air interface 116 using New Radio (NR).

In an embodiment, the base station 114 a and the WTRUs 102 a, 102 b, 102c may implement multiple radio access technologies. For example, thebase station 114 a and the WTRUs 102 a, 102 b, 102 c may implement LTEradio access and NR radio access together, for instance using dualconnectivity (DC) principles. Thus, the air interface utilized by WTRUs102 a, 102 b, 102 c may be characterized by multiple types of radioaccess technologies and/or transmissions sent to/from multiple types ofbase stations (e.g., a eNB and a gNB).

In other embodiments, the base station 114 a and the WTRUs 102 a, 102 b,102 c may implement radio technologies such as IEEE 802.11 (i.e.,Wireless Fidelity (WiFi), IEEE 802.16 (i.e., Worldwide Interoperabilityfor Microwave Access (WiMAX)), CDMA2000, CDMA2000 1×, CDMA2000 EV-DO,Interim Standard 2000 (IS-2000), Interim Standard 95 (IS-95), InterimStandard 856 (IS-856), Global System for Mobile communications (GSM),Enhanced Data rates for GSM Evolution (EDGE), GSM EDGE (GERAN), and thelike.

The base station 114 b in FIG. 1A may be a wireless router, Home Node B,Home eNode B, or access point, for example, and may utilize any suitableRAT for facilitating wireless connectivity in a localized area, such asa place of business, a home, a vehicle, a campus, an industrialfacility, an air corridor (e.g., for use by drones), a roadway, and thelike. In one embodiment, the base station 114 b and the WTRUs 102 c, 102d may implement a radio technology such as IEEE 802.11 to establish awireless local area network (WLAN). In an embodiment, the base station114 b and the WTRUs 102 c, 102 d may implement a radio technology suchas IEEE 802.15 to establish a wireless personal area network (WPAN). Inyet another embodiment, the base station 114 b and the WTRUs 102 c, 102d may utilize a cellular-based RAT (e.g., WCDMA, CDMA2000, GSM, LTE,LTE-A, LTE-A Pro, NR etc.) to establish a picocell or femtocell. Asshown in FIG. 1A, the base station 114 b may have a direct connection tothe Internet 110. Thus, the base station 114 b may not be required toaccess the Internet 110 via the CN 106/115.

The RAN 104/113 may be in communication with the CN 106/115, which maybe any type of network configured to provide voice, data, applications,and/or voice over internet protocol (VoIP) services to one or more ofthe WTRUs 102 a, 102 b, 102 c, 102 d. The data may have varying qualityof service (QoS) requirements, such as differing throughputrequirements, latency requirements, error tolerance requirements,reliability requirements, data throughput requirements, mobilityrequirements, and the like. The CN 106/115 may provide call control,billing services, mobile location-based services, pre-paid calling,Internet connectivity, video distribution, etc., and/or performhigh-level security functions, such as user authentication. Although notshown in FIG. 1A, it will be appreciated that the RAN 104/113 and/or theCN 106/115 may be in direct or indirect communication with other RANsthat employ the same RAT as the RAN 104/113 or a different RAT. Forexample, in addition to being connected to the RAN 104/113, which may beutilizing a NR radio technology, the CN 106/115 may also be incommunication with another RAN (not shown) employing a GSM, UMTS, CDMA2000, WiMAX, E-UTRA, or WiFi radio technology.

The CN 106/115 may also serve as a gateway for the WTRUs 102 a, 102 b,102 c, 102 d to access the PSTN 108, the Internet 110, and/or the othernetworks 112. The PSTN 108 may include circuit-switched telephonenetworks that provide plain old telephone service (POTS). The Internet110 may include a global system of interconnected computer networks anddevices that use common communication protocols, such as thetransmission control protocol (TCP), user datagram protocol (UDP) and/orthe internet protocol (IP) in the TCP/IP internet protocol suite. Thenetworks 112 may include wired and/or wireless communications networksowned and/or operated by other service providers. For example, thenetworks 112 may include another CN connected to one or more RANs, whichmay employ the same RAT as the RAN 104/113 or a different RAT.

Some or all of the WTRUs 102 a, 102 b, 102 c, 102 d in thecommunications system 100 may include multi-mode capabilities (e.g., theWTRUs 102 a, 102 b, 102 c, 102 d may include multiple transceivers forcommunicating with different wireless networks over different wirelesslinks). For example, the WTRU 102 c shown in FIG. 1A may be configuredto communicate with the base station 114 a, which may employ acellular-based radio technology, and with the base station 114 b, whichmay employ an IEEE 802 radio technology.

FIG. 1B is a system diagram illustrating an example WTRU 102. As shownin FIG. 1B, the WTRU 102 may include a processor 118, a transceiver 120,a transmit/receive element 122, a speaker/microphone 124, a keypad 126,a display/touchpad 128, non-removable memory 130, removable memory 132,a power source 134, a global positioning system (GPS) chipset 136,and/or other peripherals 138, among others. It will be appreciated thatthe WTRU 102 may include any sub-combination of the foregoing elementswhile remaining consistent with an embodiment.

The processor 118 may be a general purpose processor, a special purposeprocessor, a conventional processor, a digital signal processor (DSP), aplurality of microprocessors, one or more microprocessors in associationwith a DSP core, a controller, a microcontroller, Application SpecificIntegrated Circuits (ASICs), Field Programmable Gate Arrays (FPGAs)circuits, any other type of integrated circuit (IC), a state machine,and the like. The processor 118 may perform signal coding, dataprocessing, power control, input/output processing, and/or any otherfunctionality that enables the WTRU 102 to operate in a wirelessenvironment. The processor 118 may be coupled to the transceiver 120,which may be coupled to the transmit/receive element 122. While FIG. 1Bdepicts the processor 118 and the transceiver 120 as separatecomponents, it will be appreciated that the processor 118 and thetransceiver 120 may be integrated together in an electronic package orchip.

The transmit/receive element 122 may be configured to transmit signalsto, or receive signals from, a base station (e.g., the base station 114a) over the air interface 116. For example, in one embodiment, thetransmit/receive element 122 may be an antenna configured to transmitand/or receive RF signals. In an embodiment, the transmit/receiveelement 122 may be an emitter/detector configured to transmit and/orreceive IR, UV, or visible light signals, for example. In yet anotherembodiment, the transmit/receive element 122 may be configured totransmit and/or receive both RF and light signals. It will beappreciated that the transmit/receive element 122 may be configured totransmit and/or receive any combination of wireless signals.

Although the transmit/receive element 122 is depicted in FIG. 1B as asingle element, the WTRU 102 may include any number of transmit/receiveelements 122. More specifically, the WTRU 102 may employ MIMOtechnology. Thus, in one embodiment, the WTRU 102 may include two ormore transmit/receive elements 122 (e.g., multiple antennas) fortransmitting and receiving wireless signals over the air interface 116.

The transceiver 120 may be configured to modulate the signals that areto be transmitted by the transmit/receive element 122 and to demodulatethe signals that are received by the transmit/receive element 122. Asnoted above, the WTRU 102 may have multi-mode capabilities. Thus, thetransceiver 120 may include multiple transceivers for enabling the WTRU102 to communicate via multiple RATs, such as NR and IEEE 802.11, forexample.

The processor 118 of the WTRU 102 may be coupled to, and may receiveuser input data from, the speaker/microphone 124, the keypad 126, and/orthe display/touchpad 128 (e.g., a liquid crystal display (LCD) displayunit or organic light-emitting diode (OLED) display unit). The processor118 may also output user data to the speaker/microphone 124, the keypad126, and/or the display/touchpad 128. In addition, the processor 118 mayaccess information from, and store data in, any type of suitable memory,such as the non-removable memory 130 and/or the removable memory 132.The non-removable memory 130 may include random-access memory (RAM),read-only memory (ROM), a hard disk, or any other type of memory storagedevice. The removable memory 132 may include a subscriber identitymodule (SIM) card, a memory stick, a secure digital (SD) memory card,and the like. In other embodiments, the processor 118 may accessinformation from, and store data in, memory that is not physicallylocated on the WTRU 102, such as on a server or a home computer (notshown).

The processor 118 may receive power from the power source 134, and maybe configured to distribute and/or control the power to the othercomponents in the WTRU 102. The power source 134 may be any suitabledevice for powering the WTRU 102. For example, the power source 134 mayinclude one or more dry cell batteries (e.g., nickel-cadmium (NiCd),nickel-zinc (NiZn), nickel metal hydride (NiMH), lithium-ion (Li-ion),etc.), solar cells, fuel cells, and the like.

The processor 118 may also be coupled to the GPS chipset 136, which maybe configured to provide location information (e.g., longitude andlatitude) regarding the current location of the WTRU 102. In additionto, or in lieu of, the information from the GPS chipset 136, the WTRU102 may receive location information over the air interface 116 from abase station (e.g., base stations 114 a, 114 b) and/or determine itslocation based on the timing of the signals being received from two ormore nearby base stations. It will be appreciated that the WTRU 102 mayacquire location information by way of any suitablelocation-determination method while remaining consistent with anembodiment.

The processor 118 may further be coupled to other peripherals 138, whichmay include one or more software and/or hardware modules that provideadditional features, functionality and/or wired or wirelessconnectivity. For example, the peripherals 138 may include anaccelerometer, an e-compass, a satellite transceiver, a digital camera(for photographs and/or video), a universal serial bus (USB) port, avibration device, a television transceiver, a hands free headset, aBluetooth® module, a frequency modulated (FM) radio unit, a digitalmusic player, a media player, a video game player module, an Internetbrowser, a Virtual Reality and/or Augmented Reality (VR/AR) device, anactivity tracker, and the like. The peripherals 138 may include one ormore sensors, the sensors may be one or more of a gyroscope, anaccelerometer, a hall effect sensor, a magnetometer, an orientationsensor, a proximity sensor, a temperature sensor, a time sensor; ageolocation sensor; an altimeter, a light sensor, a touch sensor, amagnetometer, a barometer, a gesture sensor, a biometric sensor, and/ora humidity sensor.

The WTRU 102 may include a full duplex radio for which transmission andreception of some or all of the signals (e.g., associated withparticular subframes for both the UL (e.g., for transmission) anddownlink (e.g., for reception) may be concurrent and/or simultaneous.The full duplex radio may include an interference management unit toreduce and or substantially eliminate self-interference via eitherhardware (e.g., a choke) or signal processing via a processor (e.g., aseparate processor (not shown) or via processor 118). In an embodiment,the WRTU 102 may include a half-duplex radio for which transmission andreception of some or all of the signals (e.g., associated withparticular subframes for either the UL (e.g., for transmission) or thedownlink (e.g., for reception)).

FIG. 1C is a system diagram illustrating the RAN 104 and the CN 106according to an embodiment. As noted above, the RAN 104 may employ anE-UTRA radio technology to communicate with the WTRUs 102 a, 102 b, 102c over the air interface 116. The RAN 104 may also be in communicationwith the CN 106.

The RAN 104 may include eNode-Bs 160 a, 160 b, 160 c, though it will beappreciated that the RAN 104 may include any number of eNode-Bs whileremaining consistent with an embodiment. The eNode-Bs 160 a, 160 b, 160c may each include one or more transceivers for communicating with theWTRUs 102 a, 102 b, 102 c over the air interface 116. In one embodiment,the eNode-Bs 160 a, 160 b, 160 c may implement MIMO technology. Thus,the eNode-B 160 a, for example, may use multiple antennas to transmitwireless signals to, and/or receive wireless signals from, the WTRU 102a.

Each of the eNode-Bs 160 a, 160 b, 160 c may be associated with aparticular cell (not shown) and may be configured to handle radioresource management decisions, handover decisions, scheduling of usersin the UL and/or DL, and the like. As shown in FIG. 1C, the eNode-Bs 160a, 160 b, 160 c may communicate with one another over an X2 interface.

The CN 106 shown in FIG. 1C may include a mobility management entity(MME) 162, a serving gateway (SGW) 164, and a packet data network (PDN)gateway (or PGW) 166. While each of the foregoing elements are depictedas part of the CN 106, it will be appreciated that any of these elementsmay be owned and/or operated by an entity other than the CN operator.

The MME 162 may be connected to each of the eNode-Bs 162 a, 162 b, 162 cin the RAN 104 via an S1 interface and may serve as a control node. Forexample, the MME 162 may be responsible for authenticating users of theWTRUs 102 a, 102 b, 102 c, bearer activation/deactivation, selecting aparticular serving gateway during an initial attach of the WTRUs 102 a,102 b, 102 c, and the like. The MME 162 may provide a control planefunction for switching between the RAN 104 and other RANs (not shown)that employ other radio technologies, such as GSM and/or WCDMA.

The SGW 164 may be connected to each of the eNode Bs 160 a, 160 b, 160 cin the RAN 104 via the S1 interface. The SGW 164 may generally route andforward user data packets to/from the WTRUs 102 a, 102 b, 102 c. The SGW164 may perform other functions, such as anchoring user planes duringinter-eNode B handovers, triggering paging when DL data is available forthe WTRUs 102 a, 102 b, 102 c, managing and storing contexts of theWTRUs 102 a, 102 b, 102 c, and the like.

The SGW 164 may be connected to the PGW 166, which may provide the WTRUs102 a, 102 b, 102 c with access to packet-switched networks, such as theInternet 110, to facilitate communications between the WTRUs 102 a, 102b, 102 c and IP-enabled devices.

The CN 106 may facilitate communications with other networks. Forexample, the CN 106 may provide the WTRUs 102 a, 102 b, 102 c withaccess to circuit-switched networks, such as the PSTN 108, to facilitatecommunications between the WTRUs 102 a, 102 b, 102 c and traditionalland-line communications devices. For example, the CN 106 may include,or may communicate with, an IP gateway (e.g., an IP multimedia subsystem(IMS) server) that serves as an interface between the CN 106 and thePSTN 108. In addition, the CN 106 may provide the WTRUs 102 a, 102 b,102 c with access to the other networks 112, which may include otherwired and/or wireless networks that are owned and/or operated by otherservice providers.

Although the WTRU is described in FIGS. 1A-1D as a wireless terminal, itis contemplated that in certain representative embodiments that such aterminal may use (e.g., temporarily or permanently) wired communicationinterfaces with the communication network.

In representative embodiments, the other network 112 may be a WLAN.

A WLAN in Infrastructure Basic Service Set (BSS) mode may have an AccessPoint (AP) for the BSS and one or more stations (STAs) associated withthe AP. The AP may have an access or an interface to a DistributionSystem (DS) or another type of wired/wireless network that carriestraffic in to and/or out of the BSS. Traffic to STAs that originatesfrom outside the BSS may arrive through the AP and may be delivered tothe STAs. Traffic originating from STAs to destinations outside the BSSmay be sent to the AP to be delivered to respective destinations.Traffic between STAs within the BSS may be sent through the AP, forexample, where the source STA may send traffic to the AP and the AP maydeliver the traffic to the destination STA. The traffic between STAswithin a BSS may be considered and/or referred to as peer-to-peertraffic. The peer-to-peer traffic may be sent between (e.g., directlybetween) the source and destination STAs with a direct link setup (DLS).In certain representative embodiments, the DLS may use an 802.11e DLS oran 802.11z tunneled DLS (TDLS). A WLAN using an Independent BSS (IBSS)mode may not have an AP, and the STAs (e.g., all of the STAs) within orusing the IBSS may communicate directly with each other. The IBSS modeof communication may sometimes be referred to herein as an ‘ad-hoc’ modeof communication.

When using the 802.11ac infrastructure mode of operation or a similarmode of operations, the AP may transmit a beacon on a fixed channel,such as a primary channel. The primary channel may be a fixed width(e.g., 20 MHz wide bandwidth) or a dynamically set width via signaling.The primary channel may be the operating channel of the BSS and may beused by the STAs to establish a connection with the AP. In certainrepresentative embodiments, Carrier Sense Multiple Access with CollisionAvoidance (CSMA/CA) may be implemented, for example in in 802.11systems. For CSMA/CA, the STAs (e.g., every STA), including the AP, maysense the primary channel. If the primary channel is sensed/detectedand/or determined to be busy by a particular STA, the particular STA mayback off. One STA (e.g., only one station) may transmit at any giventime in a given BSS.

High Throughput (HT) STAs may use a 40 MHz wide channel forcommunication, for example, via a combination of the primary 20 MHzchannel with an adjacent or nonadjacent 20 MHz channel to form a 40 MHzwide channel.

Very High Throughput (VHT) STAs may support 20 MHz, 40 MHz, 80 MHz,and/or 160 MHz wide channels. The 40 MHz, and/or 80 MHz, channels may beformed by combining contiguous 20 MHz channels. A 160 MHz channel may beformed by combining 8 contiguous 20 MHz channels, or by combining twonon-contiguous 80 MHz channels, which may be referred to as an 80+80configuration. For the 80+80 configuration, the data, after channelencoding, may be passed through a segment parser that may divide thedata into two streams. Inverse Fast Fourier Transform (IFFT) processing,and time domain processing, may be done on each stream separately. Thestreams may be mapped on to the two 80 MHz channels, and the data may betransmitted by a transmitting STA. At the receiver of the receiving STA,the above described operation for the 80+80 configuration may bereversed, and the combined data may be sent to the Medium Access Control(MAC).

Sub 1 GHz modes of operation are supported by 802.11af and 802.11ah. Thechannel operating bandwidths, and carriers, are reduced in 802.11af and802.11ah relative to those used in 802.11n, and 802.11ac. 802.11afsupports 5 MHz, 10 MHz and 20 MHz bandwidths in the TV White Space(TVWS) spectrum, and 802.11ah supports 1 MHz, 2 MHz, 4 MHz, 8 MHz, and16 MHz bandwidths using non-TVWS spectrum. According to a representativeembodiment, 802.11ah may support Meter Type Control/Machine-TypeCommunications, such as MTC devices in a macro coverage area. MTCdevices may have certain capabilities, for example, limited capabilitiesincluding support for (e.g., only support for) certain and/or limitedbandwidths. The MTC devices may include a battery with a battery lifeabove a threshold (e.g., to maintain a very long battery life).

WLAN systems, which may support multiple channels, and channelbandwidths, such as 802.11n, 802.11ac, 802.11af, and 802.11ah, include achannel which may be designated as the primary channel. The primarychannel may have a bandwidth equal to the largest common operatingbandwidth supported by all STAs in the BSS. The bandwidth of the primarychannel may be set and/or limited by a STA, from among all STAs inoperating in a BSS, which supports the smallest bandwidth operatingmode. In the example of 802.11ah, the primary channel may be 1 MHz widefor STAs (e.g., MTC type devices) that support (e.g., only support) a 1MHz mode, even if the AP, and other STAs in the BSS support 2 MHz, 4MHz, 8 MHz, 16 MHz, and/or other channel bandwidth operating modes.Carrier sensing and/or Network Allocation Vector (NAV) settings maydepend on the status of the primary channel. If the primary channel isbusy, for example, due to a STA (which supports only a 1 MHz operatingmode), transmitting to the AP, the entire available frequency bands maybe considered busy even though a majority of the frequency bands remainsidle and may be available.

In the United States, the available frequency bands, which may be usedby 802.11ah, are from 902 MHz to 928 MHz. In Korea, the availablefrequency bands are from 917.5 MHz to 923.5 MHz. In Japan, the availablefrequency bands are from 916.5 MHz to 927.5 MHz. The total bandwidthavailable for 802.11ah is 6 MHz to 26 MHz depending on the country code.

FIG. 1D is a system diagram illustrating the RAN 113 and the CN 115according to an embodiment. As noted above, the RAN 113 may employ an NRradio technology to communicate with the WTRUs 102 a, 102 b, 102 c overthe air interface 116. The RAN 113 may also be in communication with theCN 115.

The RAN 113 may include gNBs 180 a, 180 b, 180 c, though it will beappreciated that the RAN 113 may include any number of gNBs whileremaining consistent with an embodiment. The gNBs 180 a, 180 b, 180 cmay each include one or more transceivers for communicating with theWTRUs 102 a, 102 b, 102 c over the air interface 116. In one embodiment,the gNBs 180 a, 180 b, 180 c may implement MIMO technology. For example,gNBs 180 a, 108 b may utilize beamforming to transmit signals to and/orreceive signals from the gNBs 180 a, 180 b, 180 c. Thus, the gNB 180 a,for example, may use multiple antennas to transmit wireless signals to,and/or receive wireless signals from, the WTRU 102 a. In an embodiment,the gNBs 180 a, 180 b, 180 c may implement carrier aggregationtechnology. For example, the gNB 180 a may transmit multiple componentcarriers to the WTRU 102 a (not shown). A subset of these componentcarriers may be on unlicensed spectrum while the remaining componentcarriers may be on licensed spectrum. In an embodiment, the gNBs 180 a,180 b, 180 c may implement Coordinated Multi-Point (CoMP) technology.For example, WTRU 102 a may receive coordinated transmissions from gNB180 a and gNB 180 b (and/or gNB 180 c).

The WTRUs 102 a, 102 b, 102 c may communicate with gNBs 180 a, 180 b,180 c using transmissions associated with a scalable numerology. Forexample, the OFDM symbol spacing and/or OFDM subcarrier spacing may varyfor different transmissions, different cells, and/or different portionsof the wireless transmission spectrum. The WTRUs 102 a, 102 b, 102 c maycommunicate with gNBs 180 a, 180 b, 180 c using subframe or transmissiontime intervals (TTIs) of various or scalable lengths (e.g., containingvarying number of OFDM symbols and/or lasting varying lengths ofabsolute time).

The gNBs 180 a, 180 b, 180 c may be configured to communicate with theWTRUs 102 a, 102 b, 102 c in a standalone configuration and/or anon-standalone configuration. In the standalone configuration, WTRUs 102a, 102 b, 102 c may communicate with gNBs 180 a, 180 b, 180 c withoutalso accessing other RANs (e.g., such as eNode-Bs 160 a, 160 b, 160 c).In the standalone configuration, WTRUs 102 a, 102 b, 102 c may utilizeone or more of gNBs 180 a, 180 b, 180 c as a mobility anchor point. Inthe standalone configuration, WTRUs 102 a, 102 b, 102 c may communicatewith gNBs 180 a, 180 b, 180 c using signals in an unlicensed band. In anon-standalone configuration WTRUs 102 a, 102 b, 102 c may communicatewith/connect to gNBs 180 a, 180 b, 180 c while also communicatingwith/connecting to another RAN such as eNode-Bs 160 a, 160 b, 160 c. Forexample, WTRUs 102 a, 102 b, 102 c may implement DC principles tocommunicate with one or more gNBs 180 a, 180 b, 180 c and one or moreeNode-Bs 160 a, 160 b, 160 c substantially simultaneously. In thenon-standalone configuration, eNode-Bs 160 a, 160 b, 160 c may serve asa mobility anchor for WTRUs 102 a, 102 b, 102 c and gNBs 180 a, 180 b,180 c may provide additional coverage and/or throughput for servicingWTRUs 102 a, 102 b, 102 c.

Each of the gNBs 180 a, 180 b, 180 c may be associated with a particularcell (not shown) and may be configured to handle radio resourcemanagement decisions, handover decisions, scheduling of users in the ULand/or DL, support of network slicing, dual connectivity, interworkingbetween NR and E-UTRA, routing of user plane data towards User PlaneFunction (UPF) 184 a, 184 b, routing of control plane informationtowards Access and Mobility Management Function (AMF) 182 a, 182 b andthe like. As shown in FIG. 1D, the gNBs 180 a, 180 b, 180 c maycommunicate with one another over an Xn interface.

The CN 115 shown in FIG. 1D may include at least one AMF 182 a, 182 b,at least one UPF 184 a,184 b, at least one Session Management Function(SMF) 183 a, 183 b, and possibly a Data Network (DN) 185 a, 185 b. Whileeach of the foregoing elements are depicted as part of the CN 115, itwill be appreciated that any of these elements may be owned and/oroperated by an entity other than the CN operator.

The AMF 182 a, 182 b may be connected to one or more of the gNBs 180 a,180 b, 180 c in the RAN 113 via an N2 interface and may serve as acontrol node. For example, the AMF 182 a, 182 b may be responsible forauthenticating users of the WTRUs 102 a, 102 b, 102 c, support fornetwork slicing (e.g., handling of different PDU sessions with differentrequirements), selecting a particular SMF 183 a, 183 b, management ofthe registration area, termination of NAS signaling, mobilitymanagement, and the like. Network slicing may be used by the AMF 182 a,182 b in order to customize CN support for WTRUs 102 a, 102 b, 102 cbased on the types of services being utilized WTRUs 102 a, 102 b, 102 c.For example, different network slices may be established for differentuse cases such as services relying on ultra-reliable low latency (URLLC)access, services relying on enhanced massive mobile broadband (eMBB)access, services for machine type communication (MTC) access, and/or thelike. The AMF 162 may provide a control plane function for switchingbetween the RAN 113 and other RANs (not shown) that employ other radiotechnologies, such as LTE, LTE-A, LTE-A Pro, and/or non-3GPP accesstechnologies such as WiFi.

The SMF 183 a, 183 b may be connected to an AMF 182 a, 182 b in the CN115 via an N11 interface. The SMF 183 a, 183 b may also be connected toa UPF 184 a, 184 b in the CN 115 via an N4 interface. The SMF 183 a, 183b may select and control the UPF 184 a, 184 b and configure the routingof traffic through the UPF 184 a, 184 b. The SMF 183 a, 183 b mayperform other functions, such as managing and allocating UE IP address,managing PDU sessions, controlling policy enforcement and QoS, providingdownlink data notifications, and the like. A PDU session type may beIP-based, non-IP based, Ethernet-based, and the like.

The UPF 184 a, 184 b may be connected to one or more of the gNBs 180 a,180 b, 180 c in the RAN 113 via an N3 interface, which may provide theWTRUs 102 a, 102 b, 102 c with access to packet-switched networks, suchas the Internet 110, to facilitate communications between the WTRUs 102a, 102 b, 102 c and IP-enabled devices. The UPF 184, 184 b may performother functions, such as routing and forwarding packets, enforcing userplane policies, supporting multi-homed PDU sessions, handling user planeQoS, buffering downlink packets, providing mobility anchoring, and thelike.

The CN 115 may facilitate communications with other networks. Forexample, the CN 115 may include, or may communicate with, an IP gateway(e.g., an IP multimedia subsystem (IMS) server) that serves as aninterface between the CN 115 and the PSTN 108. In addition, the CN 115may provide the WTRUs 102 a, 102 b, 102 c with access to the othernetworks 112, which may include other wired and/or wireless networksthat are owned and/or operated by other service providers. In oneembodiment, the WTRUs 102 a, 102 b, 102 c may be connected to a localData Network (DN) 185 a, 185 b through the UPF 184 a, 184 b via the N3interface to the UPF 184 a, 184 b and an N6 interface between the UPF184 a, 184 b and the DN 185 a, 185 b.

In view of FIGS. 1A-1D, and the corresponding description of FIGS.1A-1D, one or more, or all, of the functions described herein withregard to one or more of: WTRU 102 a-d, Base Station 114 a-b, eNode-B160 a-c, MME 162, SGW 164, PGW 166, gNB 180 a-c, AMF 182 a-b, UPF 184a-b, SMF 183 a-b, DN 185 a-b, and/or any other device(s) describedherein, may be performed by one or more emulation devices (not shown).The emulation devices may be one or more devices configured to emulateone or more, or all, of the functions described herein. For example, theemulation devices may be used to test other devices and/or to simulatenetwork and/or WTRU functions.

The emulation devices may be designed to implement one or more tests ofother devices in a lab environment and/or in an operator networkenvironment. For example, the one or more emulation devices may performthe one or more, or all, functions while being fully or partiallyimplemented and/or deployed as part of a wired and/or wirelesscommunication network in order to test other devices within thecommunication network. The one or more emulation devices may perform theone or more, or all, functions while being temporarilyimplemented/deployed as part of a wired and/or wireless communicationnetwork. The emulation device may be directly coupled to another devicefor purposes of testing and/or may performing testing using over-the-airwireless communications.

The one or more emulation devices may perform the one or more, includingall, functions while not being implemented/deployed as part of a wiredand/or wireless communication network. For example, the emulationdevices may be utilized in a testing scenario in a testing laboratoryand/or a non-deployed (e.g., testing) wired and/or wirelesscommunication network in order to implement testing of one or morecomponents. The one or more emulation devices may be test equipment.Direct RF coupling and/or wireless communications via RF circuitry(e.g., which may include one or more antennas) may be used by theemulation devices to transmit and/or receive data.

Video coding systems may be used to compress digital video signals,which may reduce the storage needs and/or the transmission bandwidth ofvideo signals. Video coding systems may include block-based,wavelet-based, and/or object-based systems. Block-based video codingsystems may include MPEG-1/2/4 part 2, H.264/MPEG-4 part 10 AVC, VC-1,High Efficiency Video Coding (HEVC) and/or Versatile Video Coding (WC).

Block-based video coding systems may include a block-based hybrid videocoding framework. FIG. 2 is a diagram of an example block-based hybridvideo encoding framework for an encoder. An encoder may include a WTRU.An input video signal 202 may be processed block-by-block. Block sizes(e.g., extended block sizes, such as a coding unit (CU)) may compresshigh resolution (e.g., 1080p and beyond) video signals. For example, aCU may include 64×64 pixels or more. A CU may be partitioned intoprediction units (PUs), and/or separate predictions may be used. For aninput video block (e.g., macroblock (MB) and/or a CU), spatialprediction 260 and/or temporal prediction 262 may be performed. Spatialprediction 260 (e.g., intra prediction) may use pixels from samples ofcoded neighboring blocks (e.g., reference samples) in the videopicture/slice to predict the current video block. The spatial prediction260 may reduce spatial redundancy, for example, that may be inherent inthe video signal. Motion prediction 262 (e.g., inter prediction and/ortemporal prediction) may use reconstructed pixels from the coded videopictures, for example, to predict the current video block. The motionprediction 262 may reduce temporal redundancy, for example, that may beinherent in the video signal. Motion prediction signals for a videoblock may be signaled by one or more motion vectors and/or may indicatethe amount and/or the direction of motion between the current blockand/or the current block's reference block. If multiple referencepictures are supported for a (e.g., each) video block, the video block'sreference picture index may be sent. The reference picture index may beused to identify from which reference picture in a reference picturestore 264 the motion prediction signal may derive.

After the spatial prediction 260 and/or motion prediction 262, a modedecision block 280 in the encoder may determine a prediction mode (e.g.,the best prediction mode), for example, based on a rate-distortionoptimization. The prediction block may be subtracted from a currentvideo block 216 and/or the prediction residual may be de-correlatedusing a transform 204 and/or a quantization 206 to achieve a bit-rate,such as a target bit rate. The quantized residual coefficients may beinverse quantized at quantization 210 and/or inverse transformed attransform 212, for example, to form the reconstructed residual, whichmay be added to the prediction block 226, for example, to form areconstructed video block. In-loop filtering (e.g., a de-blocking filterand/or adaptive loop filters) may be applied at loop filter 266 on thereconstructed video block before the reconstructed video block may beput in the reference picture store 264 and/or used to code video blocks(e.g., future video blocks). To form the output video bit-stream 220,coding mode (e.g., inter or intra), prediction mode information, motioninformation, and/or quantized residual coefficients may be sent (e.g.,may all be sent) to an entropy coding module 208, for example, to becompressed and/or packed to form the bit-stream.

FIG. 3 is a diagram of an example block-based video decoding frameworkfor a decoder. A decoder may include a WTRU. A video bit-stream 302(e.g., the video bit-stream 220 in FIG. 2 ) may be unpacked (e.g., firstunpacked) and/or entropy decoded at an entropy decoding module 308. Thecoding mode and prediction information may be sent to a spatialprediction module 360 (e.g., if intra coded) and/or to a motioncompensation prediction module 362 (e.g., if inter coded and/or temporalcoded) to form a prediction block. Residual transform coefficients maybe sent to an inverse quantization module 310 and/or to an inversetransform module 312, e.g., to reconstruct the residual block. Theprediction block and/or the residual block may be added together at 326.The reconstructed block may go through in-loop filtering at a loopfilter 366, for example, before the reconstructed block is stored in areference picture store 364. The reconstructed video 320 in thereference picture store 364 may be sent to a display device and/or usedto predict video blocks (e.g., future video blocks).

The use of bi-directional motion compensated prediction (MCP) in videocodecs may remove temporal redundancies by exploiting temporalcorrelations between pictures. A bi-prediction signal may be formed bycombining two uni-prediction signals using a weight value (e.g., 0.5).In certain videos, illuminance characteristics may change rapidly fromone reference picture to another. Thus, prediction techniques maycompensate for variations in illuminance over time (e.g., fadingtransitions) by applying global or local weights and offset values toone or more sample values in the reference pictures.

MCP in a bi-prediction mode may be performed using CU weights. As anexample, MCP may be performed using bi-prediction with CU weights. Anexample of bi-prediction with CU weights (BCW) may include generalizedbi-prediction (GBi). A bi-prediction signal may be calculated based onone or more of weight(s), motion-compensated prediction signal(s)corresponding to a motion vector associated with a reference picturelist(s), and/or the like. In an example, the prediction signal at samplex (as given) in a bi-prediction mode may be calculated using Eq.1.P[x]=w ₀ *P ₀[x+v ₀]+w ₁ *P ₁[x+v ₁]  Eq.1

P[x] may denote the resulting prediction signal of a sample x located ata picture position x. P_(i)[x+v_(i)] may denote the motion-compensatedprediction signal of x using the motion vector (MV) v_(i) for i-th list(e.g., list 0, list 1, etc.). w₀ and w₁ may denote the two weight valuesthat are applied on a prediction signal(s) for a block and/or CU. As anexample, w₀ and w₁ may denote the two weight values shared across thesamples in a block and/or CU. A variety of prediction signals may beobtained by adjusting the weight value(s). As shown in Eq.1, a varietyof prediction signals may be obtained by adjusting weight values w₀ andw₁.

Some configurations of weight values w₀ and w₁ may indicate predictionsuch as uni-prediction and/or bi-prediction. For example, (w₀, w₁)=(1,0) may be used in associated with uni-prediction with reference list L0.(w₀, w₁)=(0,1) may be used in association with uni-prediction withreference list L1. (w₀, w₁)=(0.5, 0.5) may be used in association withthe bi-prediction with two reference lists (e.g., L1 and L2).

Weight(s) may be signaled at the CU level. In an example, the weightvalues w₀ and w₁ may be signaled per CU. Bi-prediction may be performedwith the CU weights. A constraint for the weights may be applied to apair of weights. The constraint may be preconfigured. For example, theconstraint for the weights may include w₀+w₁=1. A weight may besignaled. The signaled weight may be used to determine another weight.For example, with the constraint for CU weights, only one weight may besignaled.

Signaling overhead may be reduced. Examples of pairs of weights mayinclude {(4/8, 4/8), (3/8, 5/8), (5/8, 3/8), (−2/8, 10/8), (10/8,−2/8)}.

A weight may be derived based on a constraint for the weights, forexample, when unequal weights are to be used. A coding device mayreceive a weight indication and determine a first weight based on theweight indication. The coding device may derive a second weight based onthe determined first weight and the constraint for the weights.

Eq. 2 may be used. In an example, Eq. 2 may be produced based on Eq. 1and the constraint of w₀+w₁=1.P[x]=(1−w ₁)*P ₀[x+v ₀]+w ₁ *P ₁[x+v ₁]  Eq. 2

Weight values (e.g., w₁ and/or w₀) may be discretized. Weight signalingoverhead may be reduced. In an example, bi-prediction CU weight value w₁may be discretized. The discretized weight value w₁ may include, forexample, one or more of −2/8, 2/8, 3/8, 4/8, 5/8, 6/8, 10/8, and/or thelike. A weight indication may be used to indicate weights to be used fora CU, for example, for bi-prediction. An example of the weightindication may include a weight index. In an example, each weight valuemay be indicated by an index value.

FIG. 4 is a diagram of an example video encoder with support of BCW(e.g., GBi). An encoding device as described in the example shown inFIG. 4 may be or may include a WTRU. The encoder may include a modedecision module 404, spatial prediction module 406, a motion predictionmodule 408, a transform module 410, a quantization module 412, aninverse quantization module 416, an inverse transform module 418, a loopfilter 420, a reference picture store 422 and an entropy coding module414. In examples, some or all of the encoder's modules or components(e.g., the spatial prediction module 406) may be the same as, or similarto, those described in connection with FIG. 2 . In addition, the spatialprediction module 406 and the motion prediction module 408 may bepixel-domain prediction modules. Thus, an input video bit-stream 402 maybe processed in a similar manner as the input video bit-stream 202, tooutput video bit-stream 424. The motion prediction module 408 mayfurther include support of bi-prediction with CU weights. As such, themotion prediction module 408 may combine two separate prediction signalsin a weighted-averaging manner. Further, the selected weight index maybe signaled in the input video bitstream 402.

FIG. 5 is a diagram of an example module with support for bi-predictionwith CU weights for an encoder. FIG. 5 illustrates a block diagram of anestimation module 500. The estimation module 500 may be employed in amotion prediction module of an encoder, such as the motion predictionmodule 408. The estimation module 500 may be used in connection with BCW(e.g., GBi). The estimation module 500 may include a weight valueestimation module 502 and a motion estimation module 504. The estimationmodule 500 may utilize a two-step process to generate inter predictionsignal, such as a final inter prediction signal. The motion estimationmodule 504 may perform motion estimation using reference picture(s)received from a reference picture store 506 and by searching two optimalmotion vectors (MVs) pointing to (e.g., two) reference blocks. Theweight value estimation module 502 may search for the optimal weightindex to minimize the weighted bi-prediction error between the currentvideo block and bi-prediction prediction. The prediction signal of thegeneralized bi-prediction may be computed as a weighted average of thetwo prediction blocks.

FIG. 6 is a diagram of an example block-based video decoder with supportfor bi-prediction with CU weights. FIG. 6 illustrates a block diagram ofan example video decoder that may decode a bit-stream from an encoder.The encoder may support BCW and/or share some similarities with theencoder described in connection with FIG. 4 . A decoder as described inthe example shown in FIG. 6 may include a WTRU. As shown in FIG. 6 , thedecoder may include an entropy decoder 604, a spatial prediction module606, a motion prediction module 608, a reference picture store 610, aninverse quantization module 612, an inverse transform module 614 and aloop filter module 618. Some or all of the decoder's modules may be thesame as, or similar to, those described in connection with FIG. 3 . Forexample, the prediction block and/or residual block may be addedtogether at 616. A video bit-stream 602 may be processed to generate thereconstructed video 620 that may be sent to a display device and/or usedto predict video blocks (e.g., future video blocks). The motionprediction module 608 may further include support for BCW. The codingmode and/or prediction information may be used to derive a predictionsignal using either spatial prediction or MCP with support for BCW. ForBCW, the block motion information and/or weight value (e.g., in the formof an index indicating a weight value) may be received and decoded togenerate the prediction block.

FIG. 7 is a diagram of an example module with support for bi-predictionwith CU weights for a decoder. FIG. 7 illustrates a block diagram of aprediction module 700. The prediction module 700 may be employed in amotion prediction module of a decoder, such as the motion predictionmodule 608. The prediction module 700 may be used in connection withBCW. The prediction module 700 may include a weighted averaging module702 and a motion compensation module 704, which may receive one or morereferences pictures from a reference picture store 706. The predictionmodule 700 may use the block motion information and weight value tocompute a prediction signal of BCW as a weighted average of (e.g., two)motion compensated prediction blocks.

Bi-prediction in video coding may be based on a combination of multiple(e.g., two) temporal prediction blocks. In examples, CU and block may beused interchangeably. The temporal prediction blocks may be combined. Inan example, two temporal prediction blocks that are obtained from thereference pictures that are reconstructed may be combined usingaveraging. Bi-prediction may be based on block-based motioncompensation. A relatively small motion may be observed between the(e.g., two) prediction blocks in bi-prediction.

Bi-directional optical flow (BDOF) may be used, for example, tocompensate the relatively small motion observed between predictionblocks. BDOF may be applied to compensate for such motion for a sampleinside a block. In an example, BDOF may compensate for such motion forindividual samples inside a block. This may increase the efficiency ofmotion compensated prediction.

BDOF may include refining motion vector(s) associated with a block. Inexamples, BDOF may include sample-wise motion refinement that isperformed on top of block-based motion-compensated predictions whenbi-prediction is used. BDOF may include deriving refined motionvector(s) for a sample. As an example of BDOF, the derivation of therefined motion vector for individual samples in a block may be based onthe optical flow model.

BDOF may include refining a motion vector of a sub-block associated witha block based on one or more of the following: a location in a block;gradients (e.g., horizontal, vertical, and/or the like) associated withthe location in the block; sample values associated with a correspondingreference picture list for the location; and/or the like. Eq. 3 may beused for deriving refined motion vector for a sample. As shown in Eq. 3,I^((k)) (x, y) may denote the sample value at the coordinate (x, y) ofthe prediction block, derived from the reference picture list k (k=0,1). ∂I^((k))(x, y)/∂x and ∂I^((k))(x,y)/δy may be the horizontal andvertical gradients of the sample. The motion refinement (v_(x), v_(y))at (x, y) may be derived using Eq. 3. Eq. 3 may be based on anassumption that the optical flow model is valid.

$\begin{matrix}{{\frac{\partial{I^{(k)}\left( {x,y} \right)}}{\partial t} + {v_{x} \cdot \frac{\partial{I^{(k)}\left( {x,y} \right)}}{\partial x}} + {v_{y}\frac{\partial{I^{(k)}\left( {x,y} \right)}}{\partial y}}} = 0} & {{Eq}.\mspace{14mu} 3}\end{matrix}$

FIG. 8 illustrates an example bidirectional optical flow. In FIG. 8 ,(MV_(x0), MV_(y0)) and (MV_(x1), MV_(y1)) may indicate block-levelmotion vectors. The block-level motion vectors may be used to generateprediction blocks I⁽⁰⁾ and IM. The motion refinement parameters (v_(x),v_(y)) at the sample location (x, y) may be calculated, for example, byminimizing the difference Δ between the motion vector values of thesample(s) after motion refinement (e.g., motion vector between thecurrent picture and the backward reference picture A and motion vectorbetween the current picture and the forward reference picture B in FIG.8 ). The difference Δ between the motion vector values of the samplesafter motion refinement may be calculated using, for example, Eq. 4.

$\begin{matrix}{{\Delta\left( {x,y} \right)} = {{I^{(0)}\left( {x,y} \right)} - {I^{(1)}\left( {x,y} \right)} + {v_{x}\left( {\frac{\partial{I^{(1)}\left( {x,y} \right)}}{\partial x} + \frac{\partial{I^{(0)}\left( {x,y} \right)}}{\partial x}} \right)} + {v_{y}\left( {\frac{\partial{I^{(1)}\left( {x,y} \right)}}{\partial y} + \frac{\partial{I^{(0)}\left( {x,y} \right)}}{\partial y}} \right)}}} & {{Eq}.\mspace{14mu} 4}\end{matrix}$

It may be assumed that the motion refinement is consistent for thesamples, for example inside one unit (e.g., a 4×4 block). Suchassumption may support the regularity of the derived motion refinement.The value of (v_(x)*, v_(y)*) may be derived, for example, by minimizingΔ inside the 6×6 window Ω around each 4×4 block, as shown in Eq. 5.

$\begin{matrix}{\left( {v_{x}^{*},v_{y}^{*}} \right) = {\underset{({v_{x},v_{y}})}{argmin}{\sum\limits_{{({i,j})} \in \Omega}^{\;}{\Delta^{2}\left( {i,j} \right)}}}} & {{Eq}.\mspace{14mu} 5}\end{matrix}$

In examples, BDOF may include a progressive technique which may optimizethe motion refinement in the horizontal direction (e.g., first) and inthe vertical direction (e.g., second), e.g., to be used in associationwith Eq. 5. This may result in Eq. 6.v _(x)=(S ₁ +r)>m?clip3(−th _(BIO) ,th _(BIO),−(S ₃>>└ log₂(S ₁ +r)┘)):0v _(y)=(S ₅ +r)>m?clip3(−th _(BIO) ,th _(BIO),−((S ₆ −v _(x) S ₂)≥≥└log₂(S ₅ +r)┘)):0  Eq. 6where └⋅┘ may be the floor function that outputs the greatest value thatis less than or equal to the input. th_(BIO) may be the motionrefinement value (e.g., threshold value) to prevent the errorpropagation, e.g., due to coding noise and irregular local motion. As anexample, the motion refinement value may be 2^(18−BD). The values of S₁,S₂, S₃, S₅ and S₆ may be calculated, for example, as shown in Eq. 7 andEq. 8S ₁=Σ_((i,j)∈Ω)ψ_(x)(i,j)·ψ_(x)(i,j),S ₃=Σ_((i,j)∈Ω)θ(i,j)·ψ_(x)(i,j)·2^(L)S ₂=Σ_((i,j)∈Ω)ψ_(x)(i,j)·ψ_(y)(i,j)S ₅=Σ_((i,j)∈Ω)ψ_(y)(i,j)·ψ_(y)(i,j)·2S ₆=Σ_((i,j)∈Ω)θ(i,j)·ψ_(y)(i,j)·2^(L+1)  Eq. 7where

$\begin{matrix}{{{\psi_{x}\left( {i,j} \right)} = {{\frac{\partial I^{(1)}}{\partial x}\left( {i,j} \right)} + {\frac{\partial I^{(0)}}{\partial x}\left( {i,j} \right)}}}{{\psi_{y}\left( {i,j} \right)} = {{\frac{\partial I^{(1)}}{\partial y}\left( {i,j} \right)} + {\frac{\partial I^{(0)}}{\partial y}\left( {i,j} \right)}}}{{\theta\left( {i,j} \right)} = {{I^{(1)}\left( {i,j} \right)} - {I^{(0)}\left( {i,j} \right)}}}} & {{Eq}.\mspace{14mu} 8}\end{matrix}$

The BDOF gradients in Eq. 8 in the horizontal and vertical directionsmay be obtained by calculating the difference between multipleneighboring samples at a sample position of the L0/L1 prediction block.In an example, the difference may be calculated between two neighboringsamples horizontally or vertically depending on the direction of thegradient being derived at one sample position of each L0/L1 predictionblock, for example, using Eq. 9.

$\begin{matrix}{{{\frac{\partial I^{(k)}}{\partial x}\left( {i,j} \right)} = {\left( {{I^{(k)}\left( {{i + 1},j} \right)} - {I^{(k)}\left( {{i - 1},j} \right)}} \right) \gg 4}}{{\frac{\partial I^{(k)}}{\partial y}\left( {i,j} \right)} = {\left( {{I^{(k)}\left( {i,{j + 1}} \right)} - {I^{(k)}\left( {i,{j - 1}} \right)}} \right) \gg 4}}{{k = 0},1}} & {{Eq}.9}\end{matrix}$

In Eq. 7, L may be the bit-depth increase for the internal BDOF, e.g.,to keep data precision. L may be set to 5. The regulation parameters rand m in Eq. 6 may be defined as shown in Eq. 10 (e.g., to avoiddivision by a smaller value).r=500·4^(BD−8)m=700·4^(BD−8)  Eq. 10BD may be the bit depth of the input video. The bi-prediction signal(e.g., final bi-prediction signal) of the current CU may be calculatedby interpolating the L0/L1 prediction samples along the motiontrajectory, e.g., based on the optical flow Eq. 3 and the motionrefinement derived by Eq. 6. The bi-prediction signal of the current CUmay be calculated using Eq. 11.

$\begin{matrix}{{{{pred}_{BIO}\left( {x,y} \right)} = {\left( {{I^{(0)}\left( {x,y} \right)} + {I^{(1)}\left( {x,y} \right)} + b + o_{offset}} \right) \gg {shift}}}{b = {{{rnd}\left( {\left( {v_{x}\left( {\frac{\partial{I^{(1)}\left( {x,y} \right)}}{\partial x} - \frac{\partial{I^{(0)}\left( {x,y} \right)}}{\partial x}} \right)} \right)/2^{L + 1}} \right)} + {{rnd}\left( {\left( {v_{y}\left( {\frac{\partial{I^{(1)}\left( {x,y} \right)}}{\partial y} - \frac{\partial{I^{(0)}\left( {x,y} \right)}}{\partial y}} \right)} \right)/2^{L + 1}} \right)}}}} & {{Eq}.11}\end{matrix}$shift and o_(offset) may be the offset and right shift that is appliedto combine the L0 and L1 prediction signals for bi-prediction, which maybe set equal to 15−BD and 1<<(14−BD)+2·(1<<13), respectively; rnd(.) maybe a rounding function that rounds the input value to the closestinteger value.

There may be various types of motions within a particular video, such aszoom in/out, rotation, perspective motions and other irregular motions.A translational motion model and/or an affine motion model may beapplied for MCP. The affine motion model may be four-parameter and/orsix-parameter. A first flag for (e.g., each) inter coded CU may besignaled to indicate whether the translational motion model or theaffine motion model is applied for inter prediction. If the affinemotion model is applied, a second flag may be sent to indicate whetherthe model is four-parameter or six-parameter.

The four-parameter affine motion model may include two parameters fortranslation movement in the horizontal and vertical directions, oneparameter for a zoom motion in the horizontal and vertical directionsand/or one parameter for a rotation motion in the horizontal andvertical directions. A horizontal zoom parameter may be equal to avertical zoom parameter. A horizontal rotation parameter may be equal toa vertical rotation parameter. The four-parameter affine motion modelmay be coded using two motion vectors at two control point positionsdefined at the top-left and top right corners of a (e.g., current) CU.

FIG. 9 illustrates an example four-parameter affine mode. FIG. 9illustrates an example affine motion field of a block. As shown in FIG.9 , the block may be described by two control point motion vectors (V₀,V₁). Based on a control point motion, the motion field (v_(x), v_(y)) ofone affine coded block may be described in Eq. 12.

$\begin{matrix}{{v_{x} = {{\frac{\left( {v_{1x} - v_{0x}} \right)}{w}x} - {\frac{\left( {\nu_{1y} - v_{0_{y}}} \right)}{w}y} + v_{0x}}}{v_{y} = {{\frac{\left( {v_{1y} - v_{0_{y}}} \right)}{w}x} + {\frac{\left( {v_{1x} - v_{0x}} \right)}{w}y} + v_{0y}}}} & {{Eq}.12}\end{matrix}$

In Eq. 12, (v_(0x), v_(0y)) may be a motion vector of the top-leftcorner control point. (v_(1x), v_(1y)) may be a motion vector of thetop-right corner control point. w may be the width of CU. The motionfield of an affine coded CU may be derived at a 4×4 block level. Forexample, (v_(x), v_(y)) may be derived for each of the 4×4 blocks withina current CU and applied to a corresponding 4×4 block.

The four parameters may be estimated iteratively. The motion vectorpairs at step k may be denoted as {(v_(0x) ^(k),v_(0y) ^(k)), (v_(1x)^(k),v_(1y) ^(k))}, the original luminance signal as I(i,j), and theprediction luminance signal as I′_(k)(i,j). The spatial gradient g_(x)^(k)(i,j) and g_(y) ^(k)(i,j) may be derived using a Sobel filterapplied on the prediction signal I′_(k)(i,j) in the horizontal andvertical directions, respectively. The derivative of Eq. 1 may berepresented as Eq. 13.

$\begin{matrix}\left\{ \begin{matrix}{{d{v_{x}^{k}\left( {x,y} \right)}} = {{c*x} - {d*y} + a}} \\{{{dv}_{y}^{k}\left( {x,y} \right)} = {{d*x} + {c*y} + b}}\end{matrix} \right. & {{Eq}.13}\end{matrix}$

In Eq. 13, (a, b) may be delta translational parameters and (c, d) maybe delta zoom and rotation parameters at step k. The delta MV at controlpoints may be derived with its coordinates as Eq. 14 and Eq. 15. Forexample, (0, 0), (w, 0) may be coordinates for top-left and top-rightcontrol points, respectively.

$\begin{matrix}\left\{ \begin{matrix}{{dv}_{0x}^{k} = {{v_{0x}^{k + 1} - v_{0x}^{k}} = a}} \\{{dv}_{0y}^{k} = {{v_{0y}^{k + 1} - v_{0y}^{k}} = b}}\end{matrix} \right. & {{Eq}.14}\end{matrix}$ $\begin{matrix}\left\{ \begin{matrix}{{dv}_{1x}^{k} = {\left( {v_{1x}^{k + 1} - v_{1x}^{k}} \right) = {{c*w} + a}}} \\{{dv}_{1y}^{k} = {\left( {v_{1y}^{k + 1} - v_{1y}^{k}} \right) = {{d*w} + b}}}\end{matrix} \right. & {{Eq}.15}\end{matrix}$

Based on an optical flow equation, the relationship between the changeof luminance and the spatial gradient and temporal movement may beformulated as Eq. 16.I′ _(k)(i,j)−I(i,j)=g _(x) ^(k)(i,j)*dv _(x) ^(k)(i,j)+g _(y)^(k)(i,j)*dv _(y) ^(k)(i,j)  Eq. 16

Substituting dv_(x) ^(k)(i,j) and dv_(y) ^(k)(i,j) with Eq. 13 mayproduce Eq. 17 for parameters (a, b, c, d).I′ _(k)(i,j)−I(i,j)=(g _(x) ^(k)(i,j)*i+g _(y) ^(k)(i,j)*j)*c+(−g _(x)^(k)(i,j)*j+g _(y) ^(k)(i,j)*i)*d+g _(x) ^(k)(i,j)*a+g _(y)^(k)(i,j)*b  Eq. 17

If the samples in the CU satisfy Eq. 17, the parameter set (a, b, c, d)may be derived using, for example, the least square calculation. Themotion vectors at two control points {(v_(0x) ^(k+1),v_(0y) ^(k+1)),(v_(1x) ^(k+1),v_(1y) ^(k+1))} at step (k+1) may be derived with Eqs. 14and 15, and they may be rounded to a specific precision (e.g., ¼ pel).Using the iteration, the motion vectors at two control points may berefined until it converges when parameters (a, b, c, d) may be zeros orthe iteration times meet a pre-defined limit.

The six-parameter affine motion model may include two parameters fortranslation movement in the horizontal and vertical directions, oneparameter for a zoom motion, one parameter for a rotation motion in thehorizontal direction, one parameter for a zoom motion and/or oneparameter for a rotation motion in the vertical direction. Thesix-parameter affine motion model may be coded with three motion vectorsat three control points. FIG. 10 illustrates an example six-parameteraffine mode. As shown in FIG. 10 , three control points for thesix-parameter affine coded CU may be defined at the top-left, top-rightand/or bottom left corners of CU. The motion at the top-left controlpoint may be related to a translational motion. The motion at thetop-right control point may be related to rotation and zoom motions inthe horizontal direction. The motion at the bottom-left control pointmay be related to rotation and zoom motions in the vertical direction.In the six-parameter affine motion model, the rotation and zoom motionsin the horizontal direction may not be same as those motions in thevertical direction. In an example, the motion vector of each sub-block(v_(x), v_(y)) may be derived from Eqs. 18 and 19 using three motionvectors as control points:

$\begin{matrix}{v_{x} = {v_{0x} + {\left( {v_{1x} - v_{0x}} \right)*\frac{ϰ}{w}} + {\left( {v_{2x} - v_{0x}} \right)*\frac{y}{h}}}} & {{Eq}.18}\end{matrix}$ $\begin{matrix}{v_{y} = {v_{0y} + {\left( {v_{1y} - v_{0y}} \right)*\frac{ϰ}{w}} + {\left( {v_{2y} - v_{0y}} \right)*\frac{y}{h}}}} & {{Eq}.19}\end{matrix}$

In Eqs. 18 and 19, (v_(2x), v_(2y)) may be a motion vector of thebottom-left control point. (x, y) may be a center position of asub-block. w and h may be a width and height of a CU.

The six parameters of the six-parameter affine model may be estimated,for example, in a similar way. For example, Eq. 20 may be produced basedon Eq. 13.

$\begin{matrix}\left\{ \begin{matrix}{{d{v_{x}^{k}\left( {x,y} \right)}} = {{c*x} + {d*y} + a}} \\{{d{v_{y}^{k}\left( {x,y} \right)}} = {{e*x} + {f*y} + b}}\end{matrix} \right. & {{Eq}.20}\end{matrix}$

In Eq. 20, for step k, (a, b) may be delta translation parameters. (c,d) may be delta zoom and rotation parameters for the horizontaldirection. (e, f) may be delta zoom and rotation parameters for thevertical direction. For example, Eq. 21 may be produced based on Eq. 16.I′ _(k)(i,j)−I(i,j)=(g _(x) ^(k)(i,j)*i)*c+(g _(x) ^(k)(i,j)*j)*d+(g_(y) ^(k)(i,j)*i)*e+(g _(y) ^(k)(i,j)*j)*f+g _(x) ^(k)(i,j)*a+g _(y)^(k)(i,j)*b  Eq. 21

The parameter set (a, b, c, d, e, f) may be derived using the leastsquare calculation by considering the samples within the CU. The motionvector of the top-left control point (v_(0x) ^(k+1),v_(0y) ^(k+1)) maybe calculated using Eq. 14. The motion vector of the top-right controlpoint (v_(1x) ^(k+1),v_(1y) ^(k+1)) may be calculated using Eq. 22. Themotion vector of the top-right control point (v_(2x) ^(k+1),v_(2y)^(k+1)) may be calculated using Eq. 23.

$\begin{matrix}\left\{ \begin{matrix}{{dv}_{1x}^{k} = {\left( {v_{1x}^{k + 1} - v_{1x}^{k}} \right) = {{c*w} + a}}} \\{{dv}_{1y}^{k} = {\left( {v_{1y}^{k + 1} - v_{1y}^{k}} \right) = {{e*w} + b}}}\end{matrix} \right. & {{Eq}.22}\end{matrix}$ $\begin{matrix}\left\{ \begin{matrix}{{dv}_{2x}^{k} = {\left( {v_{2x}^{k + 1} - v_{2x}^{k}} \right) = {{d*h} + a}}} \\{{dv}_{2y}^{k} = {\left( {v_{2y}^{k + 1} - v_{2y}^{k}} \right) = {{f*h} + b}}}\end{matrix} \right. & {{Eq}.23}\end{matrix}$

There may be a symmetric MV difference for bi-prediction. In someexamples, the motion vector in forward reference picture and backwardreference picture may be symmetric, e.g., due to the continuity ofmotion trajectory in bi-prediction.

SMVD may be an inter coding mode. With SMVD, the MVD of a firstreference picture list (e.g., reference picture list 1) may be symmetricto the MVD of a second reference picture list (e.g., reference picturelist 0). The motion vector coding information (e.g., MVD) of onereference picture list may be signaled, and the motion vectorinformation of another reference picture list may not be signaled. Themotion vector information of another reference picture list may bedetermined, for example, based on the signaled motion vector informationand that the motion vector information of the reference pictures listsis symmetric. In an example, the MVD of reference picture list 0 may besignaled and the MVD of List 1 may not be signaled. The MV coded withthis mode may be calculated using Eq. 24A.

$\begin{matrix}\left\{ \begin{matrix}{\left( {{mvx}_{0},{mvy}_{0}} \right) = \left( {{{mvpx}_{0} + {mvdx}_{0}},{{mvpy}_{0} + {mvdy}_{0}}} \right)} \\{\left( {{mvx}_{1},{mvy}_{1}} \right) = \left( {{{mvpx}_{1} - {mvdx}_{0}},{{mvpy}_{1} - {mvdy}_{0}}} \right)}\end{matrix} \right. & {{{Eq}.24}A}\end{matrix}$where the subscripts indicate the reference picture list 0 or 1, xindicates horizontal direction and y indicates vertical direction.

As shown in Eq. 24A, an MVD for the current coding block may indicate adifference between an MVP for the current coding block and an MV for thecurrent coding block. A person of skilled in the art would appreciatethat an MVP may be determined based on an MV of a spatial neighboringblock(s) of the current coding block and/or a temporal neighboring blockfor the current coding block. Eq. 24A may be illustrated in FIG. 11 .FIG. 11 illustrates an example non-affine motion symmetric MVD (e.g.,MVD1=−MVD0). As shown in Eq. 24A and illustrated in FIG. 11 , the MV ofa current coding block may be equal to the sum of the MVP for thecurrent coding block and the MVD of the current coding block (ornegative MVD depending on the reference picture list). As shown in Eq.24A and illustrated in FIG. 11 , the MVD of reference picture list 1(MVD1) may be equal to negative of the MVD of reference picture list 0(MVD0) for SMVD. The MV predictor (MVP) of reference picture list 0(MVP0) may or may not be symmetric to the MVP of reference picture list1 (MVP1). The MVP0 may or may not be equal to negative of the MVP1. Asshown in Eq. 24A, the MV of a current coding block may be equal to thesum of the MVP for the current coding block and the MVD of the currentcoding block. The MV of reference picture list 0 (MV0) may not be equalto negative of the MV of reference picture list 1 (MV1) based on Eq.24A. The MV of reference picture list 0 MV0 may or may not be symmetricto the MV of reference picture list 1 MV1.

SMVD may be available for bi-prediction in the case that: referencepicture list 0 includes a forward reference picture and referencepicture list 1 includes a backward reference picture; or referencepicture list 0 includes a backward reference picture and referencepicture list 1 includes a forward reference picture.

With SMVD, the reference picture indices of reference picture list 0 andlist 1 may not be signaled. They may be derived as follows. If referencepicture list 0 includes a forward reference picture and referencepicture list 1 includes a backward reference picture, the referencepicture index in list 0 may be set to the nearest forward referencepicture to the current picture, and, the reference picture index of list1 may be set to the nearest backward reference picture to the currentpicture. If reference picture list 0 includes a backward referencepicture and reference picture list 1 includes a forward referencepicture, the reference picture index in list 0 may be set to the nearestbackward reference picture to the current picture, and, the referencepicture index of list 1 may be set to the nearest forward referencepicture to the current picture.

For SMVD, a reference picture index may not need to be signaled foreither list. One set of MVD for one reference picture list (e.g. list 0)may be signaled. Signaling overhead may be reduced for bi-predictioncoding.

In a merge mode, motion information may be derived and/or be used (e.g.,directly used) to generate the prediction samples of the current CU. Amerge mode with motion vector difference (MMVD) may be used. A mergeflag may be signaled to specify whether MMVD is used for a CU. A MMVDflag may be signaled after sending a skip flag.

In MMVD, after a merge candidate is selected, the merge candidate may berefined by MVD information. The MVD information may be signaled. The MVDinformation may include one or more of a merge candidate flag, adistance index to specify motion magnitude, and/or an index forindication of motion direction. In MMVD, one of multiple candidates(e.g., the first two) candidates in the merge list may be selected to beused as the MV basis. The merge candidate flag may indicate whichcandidate is used.

The distance index may specify motion magnitude information and/or mayindicate a pre-defined offset from the starting point (e.g., from thecandidate selected to be the MV basis). FIG. 12 illustrates an examplemotion vector difference (MVD) search point(s). As shown in FIG. 12 ,the center points may be the starting point MVs. As shown in FIG. 12 ,the patterns in the dots may indicate different searching orders (e.g.,from dots nearest the center MVs to the ones further away from thecenter MVs). As shown in FIG. 12 , an offset may be added to thehorizontal component and/or vertical component of the starting point MV.Example relation of distance index and pre-defined offset is shown inTable 1.

TABLE 1 Example relation of distance index and pre-defined offsetDistance IDX 0 1 2 3 4 5 6 7 Offset (e.g., in unit ¼ ½ 1 2 4 8 16 32 ofluma sample)

The direction index may represent the direction of the MVD relative tothe starting point. The direction index may represent any of the fourdirections as shown in Table 2. The meaning of MVD sign may varyaccording to the information of the starting point MV or MVs. When thestarting point has a uni-prediction MV or a pair of bi-prediction MVswith both lists pointing to the same side of the current picture, thesign in Table 2 may specify the sign of the MV offset added to thestarting MV or MVs. For example, when the picture order counts (POCs) oftwo references are both larger than the POC of the current picture, orare both smaller than the POC of the current picture, the sign mayspecify the sign of the MV offset added to the starting MV or MVs. Whenthe starting point has a pair of bi-prediction MVs with both listspointing to the different sides of the current picture (e.g., when thePOC of one reference is larger than the POC of the current picture, andthe POC of the other reference is smaller than the POC of the currentpicture), the sign in Table 2 may specify the sign of MV offset added tothe list0 MV component of the starting point MV, and the sign of the MVoffset added to the list1 MV may have the opposite value.

TABLE 2 Example sign of MV offset specified by direction index DirectionIDX 00 01 10 11 x-axis + − N/A N/A y-axis N/A N/A + −

A symmetric mode for bi-prediction coding may be used. One or morefeatures described herein may be used in association with the symmetricmode for bi-prediction coding, e.g., which, in examples, may increasecoding efficiency and/or reduce complexity. The symmetric mode mayinclude SMVD. One or more features described herein may be associatedwith synergizing SMVD with one or more other coding tools, e.g.,bi-prediction with CU weights (BCW or BPWA), BDOF, and/or affine mode.One or more features described herein may be used in encoding (e.g.,encoder optimization), which may include fast motion estimation fortranslational and/or affine motion.

SMVD coding features may include one or more of restrictions, signaling,SMVD search features, and/or the like.

The application of SMVD mode may be based on the CU size. For example, arestriction may disallow the SMVD for a relatively small CU (e.g., a CUhaving an area that is no greater than 64). A restriction may disallowthe SMVD for a relatively large CU (e.g., a CU bigger than 32×32). If arestriction disallows the SMVD for a CU, symmetric MVD signaling may beskipped or disabled for the CU, and/or a coding device (e.g., anencoder) may not search symmetric MVD.

The application of SMVD mode may be based on the POC distances betweenthe current picture and reference pictures. The coding efficiency ofSMVD may decrease for relatively large POC distance (e.g., POC distancegreater than or equal to 8). SMVD may be disabled if the POC distancebetween a reference picture (e.g., any reference picture) to the currentpicture is relatively large. If SMVD is disabled, the symmetric MVDsignaling may be skipped or disabled, and/or a coding device (e.g., anencoder) may not search symmetric MVD.

The application of SMVD mode may be restricted to one or more temporallayers. In an example, a lower temporal layer may refer to referencepictures that have larger POC distance from the current picture, with ahierarchical GOP structure. The coding efficiency of the SMVD maydecrease for a lower temporal layer. SMVD coding may be disallowed forrelatively low temporal layers (e.g., temporal layer 0 and 1). If SMVDis disallowed, the symmetric MVD signaling may be skipped or disabled,and/or a coding device (e.g., an encoder) may not search symmetric MVD.

In SMVD coding, the MVD of one of the reference picture lists may besignaled (e.g., explicitly signaled). In an example, a coding device(e.g., a decoder) may parse a first MVD associated with a firstreference picture list in a bitstream. The coding device may determine asecond MVD associated with a second reference picture list based on thefirst MVD and that the first MVD and the second MVD are symmetric toeach other.

The coding device (e.g., the decoder) may identify the MVD of whichreference picture list is signaled, for example, whether the MVD ofreference picture list 0 or reference picture list 1 is signaled. Inexamples, the MVD of reference picture list 0 may be signaled (e.g.,always signaled). The MVD of reference picture list 1 may be obtained(e.g., derived).

The reference picture list whose MVD is signaled (e.g., explicitlysignaled) may be selected. One or more of the following may apply. Anindication (e.g., a flag) may be signaled to indicate which referencepicture list is selected. The reference picture list with a smaller POCdistance to a current picture may be selected. If the POC distances forthe reference picture lists are the same, a reference picture list maybe predetermined to break the tie. For example, reference picture list 0may be selected if the POC distances for the reference picture lists arethe same.

The MVP index for a reference picture list (e.g., one reference picturelist) may be signaled. In some examples, the indices of MVP candidatesfor both reference picture lists may be signaled (e.g., explicitlysignaled). The MVP index for a reference picture list may be signaled(e.g., only the MVP index for one reference picture list), for exampleto reduce signaling overhead. The MVP index for the other referencepicture list may be derived, for example as described herein. LX may bethe reference picture list whose MVP index is signaled (e.g., explicitlysignaled), and i may be the signaled MVP index. mvp′ may be derived fromthe MVP of LX as shown in Eq. 24.

$\begin{matrix}{\left( {{mvpx}^{\prime},{mvpy}^{\prime}} \right) = \left( {{{mvpx}_{i,{LX}} \times \frac{{POC}_{1 - {LX}} - {POC}_{curr}}{{POC}_{LX} - {POC}_{curr}}},{{mvpy}_{i,{LX}} \times \frac{{POC}_{1 - {LX}} - {POC}_{curr}}{{POC}_{LX} - {POC}_{curr}}}} \right)} & {{Eq}.24}\end{matrix}$where POC_(LX), POC_(1-LX) and POC_(curr) may be the POC of list LXreference picture, list (1-LX) reference picture, and current picture,respectively. And from the reference picture list (1-LX)'s MVP list, theMVP that is closest to mvp′ may be selected, e.g., as shown in Eq. 25,

$\begin{matrix}{j = {\arg{\min\limits_{j}\left( {{❘{{mvpx}_{j,{1 - {LX}}} - {mvpx}^{\prime}}❘} + {❘{{mvpy}_{j,{1 - {LX}}} - {mvpy}^{\prime}}❘}} \right)}}} & {{Eq}.25}\end{matrix}$where j may be the MVP index of reference picture list (1-LX). LX may bethe reference picture list whose MVD is signaled (e.g., explicitlysignaled).

Table 3 shows an example CU syntax that may support symmetric MVDsignaling for a non-affine coding mode.

TABLE 3 Example coding unit syntax that may support SMVD coding_unit(x0, y0, cbWidth, cbHeight, treeType ) { Descriptor ...    if( slice_type= = B )     inter_pred_idc[ x0 ][ y0] ae(v)    if(sps_affine_enabled_flag && cbWidth >= 16 && cbHeight >= 16 ) {    inter_affine_flag[ x0 ][ y0 ] ae(v)     if( sps_affine_type_fiag &&inter_affine_flag[ x0 ][ y0 ] )      cu_affine_type_flag[ x0 ][ y0 ]ae(v)    }    if( inter_pred_idc[ x0 ][ y0 ] == PRED_BI &&    refldxSymL0 > −1 && refldxSymL1 > −1 && inter_affine_flag[ x0 ][ y0] == 0 )     sym_mvd_flag[ x0 ][y0 ] ae(v)    if( inter_pred_idc[ x0 ][y0] != PRED_L1 ) {     if( num_ref_idx_I0_active_minus1 > 0 &&!sym_mvd_flag[ x0 ][ y0 ] )      ref_idx_I0[ x0 ][ y0 ] ae(v)    mvd_coding( x0, y0, 0, 0 )     if( MotionModelIdc[ x0 ][ y0] > 0 )     mvd_coding( x0, y0, 0, 1 )     if(MotionModelIdc[ x0 ][ y0] > 1 )     mvd_coding( x0, y0, 0, 2 )     mvp_I0_flag[ x0 ][ y0 ] ae(v)    }else {     MvdL0[ x0 ][ y0 ][ 0 ] = 0     MvdL0[ x0 ][ y0 ][ 1 ] = 0   }    if( inter_pred_idc[ x0 ][ y0 ] != PRED_L0 ) {     if(num_ref_idx_I1_active_minus1 > 0 && !sym_mvd_flag[ x0 ][ y0 ] )     ref__idx_I1[ x0 ][ y0 ] ae(v)     if( mvd_I1_zero_flag &&inter_pred_idc[ x0 ][ y0 ] = = PRED_BI ) {      ...     } else {     if( !sym_mvd_flag[ x0 ][ y0 ] ) {       mvd_coding( x0, y0, 1, 0 )     if( MotionModelIdc[ x0 ][ y0 ] > 0 )       mvd_coding( x0, y0, 1, 1)      if(MotionModelIdc[ x0 ][ y0 ] > 1 )       mvd_coding( x0, y0, 1,2 )     }     mvp_I1_flag[ x0 ][ y0 ] ae(v)    } else {     MvdL1[ x0 ][y0 ][ 0 ] = 0     MvdL1[ x0 ][ y0 ][ 1 ] = 0    }    ...   }  }  ... }

For example, an indication, such as the sym_mvd_flag flag may indicatewhether SMVD is used in motion vector coding for the current codingblock (e.g., bi-prediction coded CU).

An indication, such as refIdxSymL0, may indicate the reference pictureindex in reference picture list 0. The refIdxSymL0 indication being setto −1 may indicate that SMVD is not applicable, and the sym_mvd_flag maybe absent.

An indication, such as refIdxSymL1, may indicate the reference pictureindex in reference picture list 1. The refIdxSymL1 indication having avalue of −1 may indicate that SMVD is not applicable, and thesym_mvd_flag may be absent.

In examples, SMVD may be applicable when reference picture list 0includes a forward reference picture and reference picture list 1includes a backward reference picture, or when reference picture list 0includes a backward reference picture and reference picture list 1includes a forward reference picture. Otherwise, SMVD may not beapplicable. For example, when SMVD is not applicable, signaling of theSMVD indication (e.g., the sym_mvd_flag flag at the CU level), may beskipped. A coding device (e.g., a decoder) may perform one or morecondition checks. As shown in Table 3, two condition checks may beperformed (e.g. refIdxSymL0>−1 and refIdxSymL1>−1). The one or morecondition checks may be performed to determine whether the SMVDindication is used. As shown in Table 3, two condition checks may beperformed (e.g. refdxSymL0>−1 and refIdxSymL1>−1) to determine whetherthe SMVD indication is received. In an example, in order for a decoderto check these conditions, the decoder may wait for the construction ofthe reference picture lists of the current picture (e.g., list0 andlist1) before certain CU parsing. In some instances, even if both of thetwo checked conditions (e.g. refIdxSymL0>−1 and refIdxSymL1>−1) aretrue, an encoder may not use the SMVD for the CU (e.g., to save theencoding complexity).

An SMVD indication may be at CU level and associated with a currentcoding block. The CU-level SMVD indication may be obtained based ahigher level indication. In examples, the presence of the CU-level SMVDindication (e.g., the sym_mvd_flag flag), may be controlled (e.g.,alternatively or additionally) by a higher level indication. Forexample, a SMVD enabled indication, such as the sym_mvd_enabled_flagflag may be signaled at the slice level, tile level, tile group level,or at the picture parameter set (PPS) level, at the sequence parameterset (SPS) level, and/or in any syntax level, where the reference picturelists are shared by CUs associated with the syntax level. For example, aslice level flag can be placed in the slice header. In an example, acoding device (e.g., a decoder) may receive a sequence-level SMVDindication indicating whether SMVD is enabled for a sequence ofpictures. If SMVD is enabled for the sequence, the coding device mayobtain an SMVD indication associated with the current coding block basedon the sequence-level SMVD indication.

With the higher-level SMVD enablement indication, such as thesym_mvd_enabled_flag flag, the CU-level parsing may be performed withoutchecking the one or more conditions (e.g., described herein). The SMVDcan be enabled or disabled at the higher level than CU level (e.g. atthe discretion of the encoder). Table 4 shows example syntax that maysupport SMVD mode.

TABLE 4 Example coding unit syntax that checks the high-level SMVDenabled indication coding_unit( x0, y0, cbWidth, cbHeight, treeType ) {Descriptor ...    if( slice_type = = B )     inter_pred_idc[ x0 ][ y0 ]ae(v)    if( sps_affine_enabled_flag && cbWidth >= 16 && cbHeight >= 16) {     inter_affine_flag[ x0 ][ y0 ] ae(v)     if( sps_affine_type_flag&& inter_affine_flag[ x0 ][ y0 ] )      cu_affine_type_flag[ x0 ][ y0 ]ae(v)    }    if( inter_pred_idc[ x0 ][ y0 ] == PRED_BI &&    sym_mvd_enabled_flag && inter_affine_flag[ x0 ][ y0 ] == 0 )    sym_mvd_flag[ x0 ][ y0 ] ae(v)    if( inter_pred_idc[ x0 ][ y0 ] !=PRED_L1 ) {     if( num_ref_idx_I0_active_minus1 > 0 && !sym_mvd_flag[x0 ][ y0 ] )      ref_idx_I0[ x0 ][ y0 ] ae(v)     mvd_coding( x0, y0,0, 0 )     if( MotionModelIdc[ x0 ][ y0 ] > 0 )      mvd_coding( x0, y0,0, 1 )     if(MotionModelIdc[ x0 ][ y0 ] > 1 )      mvd_coding( x0, y0,0, 2 )     mvp_I0_flag[ x0 ][ y0 ] ae(v)    } else {     MvdL0[ x0 ][ y0][ 0 ] = 0     MvdL0[ x0 ][ y0 ][ 1 ] = 0    }    if( inter_pred_idc[ x0][ y0 ] != PRED_L0 ) {     if( num_ref_idx_I1_active_minus1 > 0 &&!sym_mvd_flag[ x0 ][ y0 ] )      ref_idx_I1[ x0 ][ y0 ] ae(v)     if(mvd_I1_zero_flag && inte_pred_idc[ x0 ][ y0] = = PRED_BI ) {      ...    } else{      if( !sym_mvd_flag[ x0 ][ y0 ] ) {       mvd_coding( x0,y0, 1, 0 )      if( MotionModelIdc[ x0 ][ y0 ] > 0 )       mvd_coding(x0, y0, 1, 1 )      if(MotionModelIdc[ x0 ][ y0 ] > 1 )      mvd_coding( x0, y0, 1, 2 )     }     mvp__I1_flag[ x0 ][ y0 ]ae(v)    } else {     MvdL1[ x0 ][ y0 ][ 0 ] = 0     MvdL1[ x0 ][ y0 ][1 ] = 0    }    ...   }  }  ... }

There may be different ways for according device (e.g., an encoder) todetermine whether to enable SMVD, and set the value of thesym_mvd_enabled_flag accordingly. One of more examples herein may becombined to determine whether to enable SMVD, and set the value of thesym_mvd_enabled_flag accordingly. For example, the encoder may determinewhether to enable SMVD by checking the reference picture lists. If bothforward and backward reference picture are present, thesym_mvd_enabled_flag flag may be set equal to true to enable SMVD. Forexample, the encoder may determine whether to enable SMVD based on thetemporal distance(s) between the current picture and the forward and/orbackward reference picture(s). When a reference picture is far away fromcurrent picture, SMVD mode may not be effective. If either the forwardor backward reference picture is far away from the current picture, theencoder may disable SMVD. A coding device may set the value of thehigh-level SMVD enablement indication based on value (e.g., a thresholdvalue) for temporal distances between a current picture and a referencepicture. For example, to reduce the encoding complexity, the encoder mayuse (e.g., only use) the higher level control flag to enable SMVD forpictures having forward and backward reference pictures such that themaximum temporal distance of these two reference pictures to the currentpicture is smaller than a value (e.g., a threshold value). For example,the encoder may determine whether to enable SMVD based on a temporallayer (e.g., of the current picture). A relatively low temporal layermay indicate that the reference picture is far away from the currentpicture, and in that case SMVD may not be effective. The encoder maydetermine that the current picture belongs to a relatively low temporallayer (e.g., lower than a threshold, such as 1, 2), and may disable SMVDfor such a current picture. For example, the sym_mvd_enabled_flag may beset to false to disable SMVD. For example, the encoder may determinewhether to enable SMVD based on the statistics of previously codedpictures at the same temporal layer as the temporal layer of the currentpicture. The statistics may include the average POC distance forbi-prediction coded CUs (e.g., the average of the distance(s) betweenthe current picture(s) and the temporal center of two reference picturesof the current picture(s)). R0, R1 may be the reference pictures forbi-prediction coded CU. poc(x) may be the POC of picture x. The POCdistance distance (CU_(i)) of two reference pictures and current picture(current_picture) may be calculated using Eq. 26. The average POCdistance for bi-prediction coded CUs AvgDist may be calculated using Eq.27.Distance(CU _(i))=|2*poc(current_picture)−poc(R0)−poc(R1)|  Eq. 26AvgDist=Σ_(CU) _(i) _(∈bi−pred)Distance(CU _(i))/N  Eq. 27Variable N may indicate the total number of bi-prediction coded CU'sthat may have both forward and backward reference pictures. As anexample, if AvgDist is smaller than a value (e.g., a predefinedthreshold), sym_mvd_enabled_flag may be set by the encoder to true toenable SMVD; otherwise, sym_mvd_enabled_flag may be set to false todisable SMVD.

In some examples, MVD value(s) may be signaled. In some examples, acombination of direction index and distance index may be signaled, andMVD value(s) may not be signaled. Example direction table and exampledistance table as shown in Table 1 and Table 2 may be used for signalingand deriving the MVD information. For example, a combination of distanceindex 0 and direction index 0 may indicate MVD (½, 0).

A search for symmetric MVD may be performed, e.g., after uni-predictionsearch and bi-prediction search. A uni-prediction search may be used tosearch for an optimal MV pointing to a reference block foruni-prediction. A bi-prediction search may be used to search for twooptimal MVs pointing to two reference blocks for bi-prediction. Thesearch may be performed to find a candidate symmetric MVD, e.g., thebest symmetric MVD. In an example, a set of search points may beiteratively evaluated, for the symmetric MVD search. An iteration mayinclude evaluation of a set of search points. The set of search pointsmay form a search pattern centered around, for example, a best MV of aprevious iteration. For the first iteration, the search pattern may becentered around the initial MV. The selection of an initial MV mayaffect overall results. A set of initial MV candidates may be evaluated.The initial MV for the symmetric MVD search may be determined, forexample, based on rate-distortion cost. In an example, the MV candidatewith the lowest rate-distortion cost may be chosen to be the initial MVfor the symmetric MVD search. The rate-distortion cost may be estimated,for example, by summing the bi-prediction error and the weighted rate ofMVD coding for reference picture list 0. The set of initial MVcandidates may include one or more of the MV(s) obtained fromuni-prediction search, the MV(s) obtained from the bi-prediction search,and the MVs from the advanced motion vector predictor (AMVP) list. Atleast one MV may be obtained for each reference picture fromuni-prediction search.

Early termination may be applied, e.g., to reduce complexity. Earlytermination may be applied (e.g., by an encoder) if bi-prediction costis larger than a value (e.g., a threshold value). In examples, e.g.,before initial MV selection, the search for symmetric MVD may beterminated if the rate-distortion cost for the MV obtained from thebi-prediction search is larger than a value (e.g., a threshold value).For example, the value may be set to a multiple of (e.g., 1.1 times) theuni-prediction cost. In examples, e.g., after initial MV selection, thesymmetric MVD search may be terminated if the rate-distortion costassociated with the initial MV is higher than a value (e.g., a thresholdvalue). For example, the value may be set to a multiple of (e.g., 1.1times of) the lowest among uni-prediction cost and bi-prediction cost.

There may be interaction between SMVD mode and other coding tools. Oneor more of the following may be performed: symmetric affine MVD coding;combine SMVD with bi-prediction with CU weights (BCW or BPWA); orcombine SMVD with BDOF.

Symmetric affine MVD coding may be used. Affine motion model parametersmay be represented by control point motion vectors. 4-parameter affinemodel may be represented by two control point MVs and 6-parameter affinemodel may be represented by three control point MVs. An example shown inEq. 12 may be a 4-parameter affine motion model represented by twocontrol point MVs, e.g., the top-left control point MV (v0) andtop-right control point MV (v). The top-left control point MV mayrepresent the translational motion. The top-left control point MV mayhave corresponding symmetric MVs, for example, associated with forwardand backward reference pictures following the motion trajectory. SMVDmode may be applied to the top-left control point. The other controlpoint MVs may represent a combination of zoom, rotate, and/or shearmapping. SMVD may not be applied to the other control point MVs.

SMVD may be applied to the top-left control point (e.g., only to thetop-left control point), and the other control point MVs may be set totheir respective MV predictor(s).

An MVD of a control point associated with a first reference picture listmay be signaled. An MVD(s) of a control point(s) associated with asecond reference picture list(s) may be obtained based on the MVD of thecontrol point associated with the first reference picture list and thatthe MVD of the control point associated with the first reference picturelist is symmetric to an MVD of a control point associated with thesecond reference picture list. In examples, when symmetric affine MVDmode is applied, the MVD of a control point associated with referencepicture list 0 may be signaled (e.g., only the control point MVD ofreference picture list 0 may be signaled). MVDs of control pointsassociated with reference picture list 1 may be derived based on thesymmetric property. The MVDs of control points associated with referencepicture list 1 may not be signaled.

The control point MVs associated with reference picture lists may bederived. The control point MVs for control point 0 (top-left) ofreference picture list 0 and reference picture list 1, may be derived,for example, using Eq. 28.

$\begin{matrix}{\left\{ \begin{matrix}{\left( {{mvx}_{0,0},{mvy}_{0,0}} \right) = \left( {{{mvpx}_{0,0} + {mvdx}_{0,0}},{{mvpy}_{0,0} + {mvdy}_{0,0}}} \right)} \\{\left( {{mvx}_{1,0},{mvy}_{1,0}} \right) = \left( {{{mvpx}_{1,0} - {mvdx}_{0,0}},{{mvpy}_{1,0} - {mvdy}_{0,0}}} \right)}\end{matrix} \right.} & {{Eq}.28}\end{matrix}$

Eq. 28 may be illustrated in FIG. 13 . FIG. 13 illustrates an exampleaffine motion symmetric MVD. As shown in Eq. 28 and illustrated in FIG.13 , the top left control point MV of a current coding block may beequal to the sum of the MVP for the top left control point of thecurrent coding block and the MVD for the top left control point of thecurrent coding block (or negative MVD depending on the reference picturelist). As shown in Eq. 28 and illustrated in FIG. 13 , the top leftcontrol point MVD associated with reference picture list 1 may be equalto negative of top left control point MVD associated with referencepicture list 0 for symmetric affine MVD coding.

The MVs for other control points may be derived using at least theaffine MVP prediction, for example, as shown in Eq. 29.

$\begin{matrix}{\left\{ \begin{matrix}{\left( {{mvx}_{0,j},{mvy}_{0,j}} \right) = \left( {{{mvpx}_{0,j} + {mvdx}_{0,j} + {mvdx}_{0,0}},{{mvpy}_{0,j} + {mvdy}_{0,j} + {mvdy}_{0,0}}} \right)} \\{\left( {{mvx}_{1,j},{mvy}_{1,j}} \right) = \left( {{{mvpx}_{1,j} - {mvdx}_{0,0}},{{mvpy}_{1,j} - {mvdx}_{0,0}}} \right)}\end{matrix} \right.} & {{Eq}.29}\end{matrix}$In Eqs. 28-30, the first dimension of the subscripts may indicate thereference picture list. The second dimension of the subscripts mayindicate the control point index.

The translational MVD (e.g., mvdx_(0,0), mvdy_(0,0)) of a firstreference picture list may be applied to the top-left control point MVderivation of a second reference picture list (e.g., mvx_(1,0),mvy_(1,0)). The translational MVD (e.g., mvdx_(0,0), mvdy_(0,0)) of thefirst reference picture list may not be applied to other control pointMV derivation of the second reference picture list (e.g., mvx_(1,j),mvy_(1,j)). In some examples of symmetric affine MVD derivation, thetranslational MVD (mvdx_(0,0), mvdy_(0,0)) of reference picture list 0may be applied (e.g., only applied) to the top-left control point MVderivation of reference picture list 1. The other control-point MVs oflist 1 may be the same as the corresponding predictor(s), for example,as shown in Eq. 30.

$\begin{matrix}{\left\{ \begin{matrix}{\left( {{mvx}_{0,j},{mvy}_{0,j}} \right) = \left( {{{mvpx}_{0,j} + {mvdx}_{0,j} + {mvdx}_{0,0}},{{mvpy}_{0,j} + {mvdy}_{0,j} + {mvdy}_{0,0}}} \right)} \\{\left( {{mvx}_{1,j},{mvy}_{1,j}} \right) = \left( {{mvpx}_{1,j},{mvpy}_{1,j}} \right)}\end{matrix} \right.} & {{Eq}.30}\end{matrix}$Table 5 shows example syntax that may be used for signaling the use ofSMVD in combination with affine mode (e.g., symmetric affine MVDcoding).

TABLE 5 Coding unit syntax for combined SMVD and affine modecoding_unit( x0, y0, cbWidth, cbHeight, treeType ) { Descriptor ...   if( slice_type = = B )     inter_pred_idc[ x0 ][ y0 ] ae(v)    if(sps_affine_enabled_flag && cbWidth >= 16 && cbHeight >= 16 ) {    inter_affine_flag[ x0 ][ y0 ] ae(v)     if( sps__affine__type_flag&& inter_affine_flag[ x0 ][ y0 ] )      cu_affine_type_flag[ x0 ][y0 ]ae(v)    }    if( inter_pred_idc[ x0 ][ y0 ] == PRED_BI &&sym_mvd_enabled_flag)     sym_mvd_flag[ x0 ][ y0 ] ae(v)    if(inter_pred_idc[ x0 ][ y0 ] != PRED_L1 ) {     if(num_ref_idx_I0_active_minus1 > 0 && !sym_mvd__flag[ x0 ][ y0 ] )     ref_idx_I0[ x0 ][ y0 ] ae(v)     mvd_coding( x0, y0, 0, 0 )     if(MotionModelIdc[ x0 ][ y0 ] > 0 )      mvd_coding( x0, y0, 0, 1 )    if(MotionModelIdc[ x0 ][ y0 ] > 1 )      mvd_coding( x0, y0, 0, 2 )    mvp_I0_flag[ x0 ][ y0 ] ae(v)    } else {     MvdL0[ x0 ][ y0 ][ 0 ]= 0     MvdL0[ x0 ][ y0 ][ 1 ] = 0    }    if( inter_pred_idc[ x0 ][ y0] != PRED_L0 ) {     if( num_ref_idx_I1_active_minus1 > 0 &&!sym_mvd_flag[ x0 ][ y0 ] )      ref_idx_I1[ x0 ][ y0 ] ae(v)     if(mvd_I1_zero_flag && inter_pred_idc[ x0 ][ y0 ] = = PRED_BI ) {      ...    } else {      if( !sym_mvd_flag[ x0 ][ y0 ] ) {       mvd_coding(x0, y0, 1, 0 )      if( MotionModelIdc[ x0 ][ y0 ] > 0 )      mvd_coding( x0, y0, 1, 1 )      if(MotionModelIdc[ x0 ][ y0 ] > 1)       mvd_coding( x0, y0, 1, 2 )     }     mvp_I1_flag[ x0 ][ y0 ]ae(v)    } else {     MvdL1[ x0 ][ y0 ][ 0 ] = 0     MvdL1[ x0 ][ y0 ][1 ] = 0    }    ...   }  }  ... }

Whether affine SMVD is used may be determined, for example, based on aninter-affine indication and/or an SMVD indication. For example, wheninter-affine indication inter_affine_flag is 1 and SMVD indicationsym_mvd_flag[x0][y0] is 1, affine SMVD may be applied. When inter-affineindication inter_affine_flag is 0 and SMVD indicationsym_mvd_flag[x0][y0] is 1, non-affine motion SMVD may be applied. IfSMVD indication sym_mvd_flag[x0][y0] is 0, SMVD may not be applied.

The MVDs of the control points in a reference picture list (e.g.,reference picture list 0) may be signaled. In an example, the MVDvalue(s) of a subset of the control points in a reference picture listmay be signaled. For example, the MVD of the top-left control point inreference picture list 0 may be signaled. The signaling of the MVDs ofthe other control points in reference picture list 0 may be skipped,and, for example, the MVDs of these control points may be set to 0. TheMVs of the other control points may be derived based on the MVD of thetop-left control point in reference picture list 0. For example, the MVsof the control points, whose MVDs are not signaled, may be derived asshown in Eq. 31.

$\begin{matrix}\left\{ \begin{matrix}{\left( {{mvx}_{0,j},{mvy}_{0,j}} \right) = \left( {{{mvpx}_{0,j} + {mvdx}_{0,0}},{{mvpy}_{0,j} + {mvdy}_{0,0}}} \right)} \\{\left( {{mvx}_{1,j},{mvy}_{1,j}} \right) = \left( {{{mvpx}_{1,j} - {mvdx}_{0,0}},{{mvpy}_{1,j} - {mvdx}_{0,0}}} \right)}\end{matrix} \right. & {{Eq}.31}\end{matrix}$

Table 6 shows example syntax that may be used for signaling theinformation related to SMVD mode in combination with affine mode.

TABLE 6 Example coding unit syntax for combined SMVD and affine modecoding_unit( x0, y0, cbWidth, cbHeight, treeType ) { Descriptor ...   if( slice_type = = B )     inter_pred_idc[ x0 ][ y0 ] ae(v)    if(sps_affine_enabled_flag && cb)Width >= 16 && cbHeight >= 16 ) {    inter_affine_flag[ x0 ][ y0 ] ae(v)     if( sps_affine_type_flag &&inter_affine_flag[ x0 ][ y0 ] )      cu_affine_type_flag[ x0 ][ y0 ]ae(v)    }    if( inter_pred_idc[ x0 ][ y0 ] == PRED_BI &&sym_mvd_enabled_flag)     sym_mvd_flag[ x0 ][ y0 ] ae(v)    if(inter_pred_idc[ x0 ][ y0 ] != PRED_L1 ) {     if(num_ref_idx_I0_active_minus1 > 0 && !sym_mvd_flag[ x0 ][ y0 ] )     ref_idx_I0[ x0 ][ y0 ] ae(v)     mvd_coding( x0, y0, 0, 0 )     if(MotionModelIdc[ x0 ][ y0 ] > 0 && !sym_mvd_flag[ x0 ][ y0 ])     mvd_coding( x0, y0, 0, 1 )     if(MotionModelIdc[ x0 ][ y0 ] > 1 &&!sym_mvd_flag[ x0 ][ y0 ])      mvd_coding( x0, y0, 0, 2 )    mvp_I0_flag[ x0 ][ y0 ] ae(v)    } else {     MvdL0[ x0 ][ y0 ][ 0 ]= 0     MvdL0[ x0 ][ y0 ][ 1 ] = 0    }    if( inter_pred_idc[ x0 ][ y0] != PRED_L0 ) {     if( num_ref_idx_I1_active_minus1 > 0 &&!sym_mvd_flag[ x0 ][ y0 ] )      ref_idx_I1 [ x0 ][ y0 ] ae(v)     if(mvd_I1_zero_flag && inter_pred_idc[ x0 ][ y0 ] = = PRED_B1 ) {      ...    } else {      if( !sym_mvd_flag[ x0 ][ y0 ] ) {       mvd_coding(x0, y0, 1, 0 )      if( MotionModelIdc[ x0 ][ y0 ] > 0 )      mvd_coding( x0, y0, 1, 1 )      if(MotionModelIdc[ x0 ][ y0 ] > 1)       mvd_coding( x0, y0, 1, 2 )     }     mvp_I1_flag[ x0 ][ y0 ]ae(v)    } else {     MvdL1[ x0 ][ y0 ][ 0 ] = 0     MvdL1[ x0 ][ y0 ][1 ] = 0    }    ...   }  }  ... }

An indication, such as a top left MVD only flag, may indicate whetheronly the MVD of the top-left control point in a reference picture list(e.g., reference picture list 0) is signaled, or whether the MVDs of thecontrol points in the reference picture list are signaled. Thisindication may be signaled at the CU level. Table 7 shows example syntaxthat may be used for signaling the information related to SMVD mode incombination with affine mode.

TABLE 7 Example coding unit syntax for combined SMVD and affine modecoding_unit( x0, y0, cbWidth, cbHeight, treeType ) { Descriptor ...   if( slice_type = = B )     inter_pred_idc[ x0 ][ y0 ] ae(v)    if(sps_affine_enabled_flag && cbWidth >= 16 && cbHeight >= 16 ) {    inter_affine_flag[ x0 ][ y0 ] ae(v)     if( sps_affine_type_flag &&inter_affine_flag[ x0 ][ y0 ] )      cu_affine_type_flag[ x0 ][ y0 ]ae(v)    }    if( inter_pred_idc[ x0 ][ y0 ] == PRED_BI &&sym_mvd_enabled_flag)     sym_mvd_flag[ x0 ][ y0 ] ae(v)    if(inter_pred_idc[ x0 ][ y0 ] == PRED_BI && sym_mvd_enabled_flag)    top_left_mvd_only_flag[ x0 ][ y0 ] ae(v)    if( inter_pred_idc[ x0][ y0 ] != PRED_L1 ) {     if( num_ref_idx_I0_active_minus1 > 0 &&!sym_mvd_flag[ x0 ][ y0 ] )      ref_Idx_I0[ x0 ][ y0 ] ae(v)    mvd_coding( x0, y0, 0, 0 )     if( MotionModelIdc[ x0 ][ y0 ] > 0 &&!sym_mvd_flag[ x0 ][ y0 ]      && ! top_left_mvd_only_flag[ x0 ][ y0 ])     mvd_coding( x0, y0, 0, 1 )     if(MotionModelIdc[ x0 ][ y0 ] > 1&&!sym_mvd_flag[ x0 ][ y0 ]      && ! topleft_mvd_only_flag[ x0 ][ y0 ])     mvd_coding( x0, y0, 0, 2 )     mvp_I0_flag[ x0 ][ y0 ] ae(v)    }else {     MvdL0[ x0 ][ y0 ][ 0 ] = 0     MvdL0[ x0 ][ y0 ][ 1 ] = 0   }    if( inter_pred_idc[ x0 ][ y0 ] != PRED_L0 ) {     if(num_ref_idx_I1_active_minus1 > 0 && !sym_mvd_flag[ x0 ][ y0 ] )     ref_Idx_I1[ x0 ][ y0 ] ae(v)     if( mvd_I1_zero_flag &&inter_pred_idc[ x0 ][ y0] = = PRED_BI ) {      ...     } else {      if(!sym_mvd_flag[ x0 ][ y0 ] ) {       mvd_coding( x0, y0, 1, 0 )      if(MotionModelIdc[ x0 ][ y0 ] > 0 )       mvd_coding( x0, y0, 1, 1 )     if(MotionModelIdc[ x0 ][ y0 ] > 1 )       mvd_coding( x0, y0, 1, 2)     }     mvp_I1_flag[ x0 ][ y0 ] ae(v)    } else {     MvdL1[ x0 ][y0 ][ 0 ] = 0     MvdL1[ x0 ][ y0 ][ 1 ] = 0    }    ...   }  }  ... }

In examples, a combination of direction index and distance index, asdescribed herein with respect to MMVD, may be signaled. Exampledirection table and example distance table are shown in Table 1 andTable 2. For example, a combination of distance index 0 and directionindex 0 may indicate MVD (½, 0).

In examples, an indication (e.g., a flag) may be signaled to indicatewhich reference picture list's MVDs are signaled. The other referencepicture list's MVDs may not be signaled; they may be derived.

One or more restrictions described herein for translational motionsymmetric MVD coding may be applied to affine motion symmetric MVDcoding, for example, to reduce complexity and/or reduce signalingoverhead.

With the symmetric affine MVD mode, signaling overhead may be reduced.Coding efficiency may be improved.

Bi-predictive motion estimation may be used to search for a symmetricMVD for an affine model. In an example, to find a symmetric MVD foraffine model (e.g., the best symmetric MVD for affine model), thebi-predictive motion estimation may be applied after uni-predictionsearch. The reference pictures for reference picture list 0 and/orreference picture list 1 may be derived as described herein. The initialcontrol point MVs may be selected from one or more of the uni-predictionsearching results, bi-prediction searching results, and/or the MVs fromthe affine AMVP list. A control point MV (e.g., the control point MVwith the lowest rate-distortion cost) may be selected to be the initialMV. A coding device (e.g., the encoder) may check one or more cases: ina first case, symmetric MVD may be signaled for reference picture list0, and control point MVs for reference picture list 1 may be derivedbased on symmetric mapping (using Eq. 28 and/or Eq. 29); in a secondcase, symmetric MVD for reference picture list 1 may be signaled, andcontrol point MVs for reference picture list 0 may be derived based onsymmetric mapping. The first case may be used as an example herein. Thesymmetric MVD search technique may be applied based on uni-predictionsearching results. Given the control point MV predictors in referencepicture list 1, an iterative search using a pre-defined search pattern(e.g., diamond pattern, cubic pattern, and/or the like) may be applied.At an iteration (e.g., each iteration), the MVD may be refined by thesearch pattern, and the control point MVs in reference picture list 0and reference picture list 1 may be derived using Eq. 28 and/or Eq. 29.The bi-prediction error corresponding to control point MVs of referencepicture list 0 and reference picture list 1 may be evaluated. Therate-distortion cost may be estimated, for example, by summing thebi-prediction error and the weighted rate of MVD coding for referencepicture list 0. In an example, the MVD with a low (e.g., the lowest)rate-distortion cost during the searching candidates may be treated asthe best MVD for the symmetric MVD search process. The other controlpoint MVs such as top-right and bottom-left control point MVs may berefined, e.g., using the optical flow-based technique described herein.

Symmetric MVD search for symmetric affine MVD coding may be performed.In an example, a set of parameters such as translational parameters maybe searched first, followed by non-translational parameters searching.In an example, optical flow search may be performed by (e.g., jointly)considering the MVD of reference picture list 0 and reference picturelist 1. For 4-parameter affine model, an example optical flow equationfor the list 0 MVDs may be shown in Eq. 32.I _(k) ⁰′(i,j)−I(i,j)=(g _(x) ⁰(i,j)−i+g _(y) ⁰(i,j)·j)·c+(−g _(x)⁰(i,j)·j+g _(y) ⁰(i,j)·i)·d+g _(x) ⁰(i,j)·a+g _(y) ⁰(i,j)·b  Eq. 32where I_(k) ⁰′ may denote the list 0 prediction in the k-th iteration,and g_(x) ⁰ and g_(y) ⁰ may denote spatial gradients of the list 0prediction.

Reference picture list 1 may have translation change (e.g., may havetranslation change only). The translation change may have the samemagnitude as that of reference picture list 0 but in a reversedirection, which is the condition of symmetric affine MVD. An exampleoptical flow equation for reference picture list 1 MVDs may be shown inEq. 33.I _(k) ¹′(i,j)−I(i,j)=−g _(x) ¹(i,j)·a−g _(y) ¹(i,j)·b  Eq. 33

The BCW weights w₀ and w₁ to list 0 prediction and list 1 prediction maybe applied respectively. An example optical equation for the symmetricaffine model may be shown in Eq. 34,I _(k)′(i,j)−I(i,j)=(G _(x)(i,j)·i+G _(y)(i,j)·j)·c+(−G _(x)(i,j)·j+G_(y)(i,j)·i)·d+H _(x)(i,j)·a+H _(y)(i,j)·b  Eq. 34whereI _(k)′(i,j)=w ₀ ·I _(k) ⁰′(i,j)+w ₁ ·I _(k) ¹′(i,j)G _(x)(i,j)=g _(x) ⁰ ·w ₀G _(y)(i,j)=g _(y) ⁰ ·w ₀H _(x)(i,j)=g _(x) ⁰ ·w ₀ −g _(x) ¹ ·w ₁H _(y)(i,j)=g _(y) ⁰ ·w ₀ −g _(y) ¹ ·w ₁

Parameters a, b, c, d may be estimated (e.g., by the least mean squareerror calculation).

An example optical flow equation for symmetric 6-parameter affine modelmay be shown in Eq. 35.I _(k)′(i,j)−(i,j)=G _(x)(i,j)·i·c+G _(x)(i,j)·j·d+G _(y)(i,j)·i·e+G_(y)(i,j)·j·f+H _(x)(i,j)·a+H _(y)(i,j)·b  Eq. 35

Parameters a, b, c, d, e, f may be estimated by the least mean squareerror calculation. When joint optical-flow motion search is performed,the affine parameters may be optimized jointly. Performance may beimproved.

Early termination may be applied, e.g., to reduce complexity. Inexamples, e.g., before initial MV selection, the search may beterminated if the bi-prediction cost is larger than a value (e.g., athreshold value). For example, the value can be set to a multiple of thecost of uni-prediction cost, e.g., 1.1 times the cost of uni-predictioncost. In examples, a coding device (e.g., the encoder) may, before theME of symmetric affine MVD starts, compare the current best affinemotion estimation (ME) cost (considering uni-prediction andbi-prediction affine search) with the non-affine ME cost. If the currentbest affine ME cost is larger than the non-affine ME cost multiplied bya value (e.g., a threshold value such as 1.1), the coding device mayskip the ME of symmetric affine MVD. In examples, e.g., after initial MVselection, the affine symmetric MVD search may be skipped if the cost ofthe initial MV is higher than a value (e.g., a threshold value). Forexample, the value may be a multiple (e.g., set to 1.1 times) of thelowest among uni-prediction cost and bi-prediction cost. In examples,the value may be set to a multiple (e.g., 1.1) times of the non-affineME cost.

SMVD mode may be combined with BCW. When the BCW is enabled for currentCU, SMVD may be applied in one or more ways. In certain examples, SMVDmay be enabled when (e.g., only when) the BCW weight is equal weight(e.g., 0.5); and, SMVD may be disabled for other BCW weights. In such acase, the SMVD flag may be signaled before BCW weight index, and thesignaling of BCW weight index may be conditionally controlled by theSMVD flag. When SMVD indication (e.g., SMVD flag) may have a value of 1,the signaling of BCW weight index may be skipped. The decoder may inferthe SMVD indication as having a value 0, which may correspond to theequal weight for bi-predictive averaging. When the SMVD flag is 0, theBCW weight index may be coded for bi-prediction mode. In the examplewhere SMVD may be enabled when the BCW weight is equal weight anddisabled for other BCW weights, the BCW index may sometimes be skipped.In some examples, the SMVD may be fully combined with BCW. The SMVD flagand BCW weight index may be signaled for explicit bi-prediction mode.The MVD search (e.g., of the encoder) for SMVD may consider the BCWweight index during bi-predictive averaging. The SMVD search may bebased on an evaluation of one or more (e.g., all) possible BCW weights.

A coding tool (e.g., bi-directional optical flow (BDOF)) may be used inassociation with one or more other coding tools/modes. BDOF may be usedin association with SMVD. Whether BDOF is applied for a coding block maydepend on whether SMVD is used. SMVD may be based on the assumption ofsymmetric MVD at a coding block level. BDOF, if performed, may be usedto refine sub-block MVs based on optical flow. Optical flow may be basedon the assumption of symmetric MVD at a sub-block level.

BDOF may be used in association with SMVD. In an example, a codingdevice (e.g., a decoder or an encoder) may receive one or moreindications that SMVD and/or BDOF are enabled. BDOF may be enabled forthe current picture. The coding device may determine whether BDOF is tobe bypassed or performed on a current coding block. The coding devicemay determine whether to bypass BDOF based on an SMVD indication (e.g.,sym_mvd_flag[x0][y0]). BDOF may be used interchangeably with BIO in someexamples.

The coding device may determine whether to bypass BDOF for a currentcoding block. BDOF may be bypassed on the current coding block if SMVDmode is used for motion vector coding of the current coding block, e.g.,to reduce decoding complexity. If SMVD is not used for the motion vectorcoding of the current coding block, the coding device may determinewhether to enable BDOF for the current coding block, for example, basedon at least another condition.

The coding device may obtain an SMVD Indication (e.g.,sym_mvd_flag[x0][y0]). The SMVD indication may indicate whether SMVD isused in motion vector coding for the current coding block.

The current coding block may be reconstructed based on the determinationof whether to bypass BDOF. An MVD may be signaled (e.g., explicitlysignaled) at the CU level for the SMVD mode.

The coding device may be configured to perform motion vector codingusing SMVD without BDOF based on the determination to bypass BDOF.

Although features and elements are described above in particularcombinations, one of ordinary skill in the art will appreciate that eachfeature or element can be used alone or in any combination with theother features and elements. In addition, the methods described hereinmay be implemented in a computer program, software, or firmwareincorporated in a computer-readable medium for execution by a computeror processor. Examples of computer-readable media include electronicsignals (transmitted over wired or wireless connections) andcomputer-readable storage media. Examples of computer-readable storagemedia include, but are not limited to, a read only memory (ROM), arandom access memory (RAM), a register, cache memory, semiconductormemory devices, magnetic media such as internal hard disks and removabledisks, magneto-optical media, and optical media such as CD-ROM disks,and digital versatile disks (DVDs). A processor in association withsoftware may be used to implement a radio frequency transceiver for usein a WTRU, UE, terminal, base station, RNC, or any host computer.

What is claimed is:
 1. A video processing device comprising: a processorconfigured to: determine that bi-directional optical flow (BDOF) isenabled for a picture comprising a current coding block; obtain asymmetric motion vector difference (SMVD) indication associated with thecurrent coding block, the SMVD indication indicating whether SMVD isused in a determination of a motion vector (MV) for the current codingblock; determine whether to bypass BDOF for the current coding blockbased on the SMVD indication associated with the current coding block,wherein, the processor is configured to determine, on a condition thatSMVD is used in the determination of the MV for the current codingblock, to bypass BDOF for the current coding block; and reconstruct thecurrent coding block based on the determination of whether to bypassBDOF.
 2. The video processing device of claim 1, wherein SMVD uses amotion vector difference (MVD) for the current coding block, and the MVDindicates a difference between a motion vector predictor (MVP) for thecurrent coding block and the MV for the current coding block.
 3. Thevideo processing device of claim 2, wherein the MVP for the currentcoding block is determined based on an MV of a spatial neighboring blockof the current coding block or a temporal neighboring block of thecurrent coding block.
 4. The video processing device of claim 1,wherein, on a condition that the SMVD indication indicates SMVD is usedin the determination of the MV for the current coding block, theprocessor is configured to: receive a first MV information associatedwith a first reference picture list; and determine a second MVinformation associated with a second reference picture list based on thefirst MV information associated with the first reference picture list,wherein a motion vector difference (MVD) associated with the firstreference picture list and an MVD associated with the second referencepicture list are symmetric.
 5. The video processing device of claim 1,wherein, on a condition that the SMVD indication indicates SMVD is usedin the determination of the MV for the current coding block, theprocessor is configured to: parse a first motion vector difference (MVD)associated with a first reference picture list in video data; anddetermine a second MVD associated with a second reference picture listbased on the first MVD, wherein the first MVD and the second MVD aresymmetric to each other.
 6. The video processing device of claim 1,wherein based on a determination not to bypass BDOF for the currentcoding block, the processor is configured to: refine an MV associatedwith a sub-block of the current coding block based at least in part ongradients associated with a location in the current coding block.
 7. Thevideo processing device of claim 1, wherein the processor is configuredto receive a sequence-level SMVD indication indicating whether SMVD isenabled for a sequence of pictures, the current coding block beingcomprised in the picture that is in the sequence, wherein, on acondition that SMVD is enabled for the sequence, the processor isconfigured to obtain the SMVD indication associated with the currentcoding block based on the sequence-level SMVD indication.
 8. The videoprocessing device of claim 1, wherein reconstructing the current codingblock based on the determination of whether to bypass BDOF comprisesreconstructing the current coding block without performing BDOF based onthe determination to bypass BDOF.
 9. A method comprising: determiningthat bi-directional optical flow (BDOF) is enabled for a picturecomprising a current coding block; obtaining a symmetric motion vectordifference (SMVD) indication associated with the current coding block,the SMVD indication indicating whether SMVD is used in a determinationof a motion vector (MV) for the current coding block; determiningwhether to bypass BDOF for the current coding block based on the SMVDindication associated with the current coding block, wherein, on acondition that SMVD is used in the determination of the MV for thecurrent coding block, the method comprises bypassing BDOF for thecurrent coding block; and reconstructing the current coding block basedon the determination of whether to bypass BDOF.
 10. The method of claim9, wherein SMVD uses a motion vector difference (MVD) for the currentcoding block, and the MVD indicates a difference between a motion vectorpredictor (MVP) for the current coding block and the MV for the currentcoding block.
 11. The method of claim 10, wherein the MVP for thecurrent coding block is determined based on an MV of a spatialneighboring block of the current coding block or a temporal neighboringblock of the current coding block.
 12. The method of claim 9, wherein,on a condition that the SMVD indication indicates SMVD is used in thedetermination of the MV for the current coding block, the methodcomprises: receiving a first MV information associated with a firstreference picture list; and determining a second MV informationassociated with a second reference picture list based on the first MVinformation associated with the first reference picture list, wherein amotion vector difference (MVD) associated with the first referencepicture list and an MVD associated with the second reference picturelist are symmetric.
 13. The method of claim 9, wherein, on a conditionthat the SMVD indication indicates SMVD is used in the determination ofthe MV for the current coding block, the method comprises: parsing afirst motion vector difference (MVD) associated with a first referencepicture list in video data; and determining a second MVD associated witha second reference picture list based on the first MVD, wherein thefirst MVD and the second MVD are symmetric to each other.
 14. The methodof claim 9, wherein, based on a determination not to bypass BDOF for thecurrent coding block, the method comprises: refining an MV associatedwith a sub-block of the current coding block based at least in part ongradients associated with a location in the current coding block. 15.The method of claim 9, comprising receiving a sequence-level SMVDindication indicating whether SMVD is enabled for a sequence ofpictures, the current coding block being comprised in the picture thatis in the sequence, wherein, on a condition that SMVD is enabled for thesequence, the method comprises obtaining the SMVD indication associatedwith the current coding block based on the sequence-level SMVDindication.
 16. The method of claim 9, wherein reconstructing thecurrent coding block based on the determination of whether to bypassBDOF further comprises reconstructing the current coding block withoutperforming BDOF based on the determination to bypass BDOF.
 17. Themethod of claim 9, wherein the method is performed by a decoder.
 18. Themethod of claim 9, wherein the method is performed by an encoder. 19.The video processing device of claim 1, wherein the video processingdevice comprises a memory.
 20. A non-transitory computer-readable mediumincluding instructions for causing one or more processors to: determinethat bi-directional optical flow (BDOF) is enabled for a picturecomprising a current coding block; obtain a symmetric motion vectordifference (SMVD) indication associated with the current coding block,the SMVD indication indicating whether SMVD is used in a determinationof a motion vector (MV) for the current coding block; determine whetherto bypass BDOF for the current coding block based on the SMVD indicationassociated with the current coding block, wherein, on a condition thatSMVD is used in the determination of the MV for the current codingblock, BDOF is determined to be bypassed for the current coding block;and reconstruct the current coding block based on the determination ofwhether to bypass BDOF.
 21. The non-transitory computer-readable mediumof claim 20, further comprising instructions for causing the one or moreprocessors to, based on a condition that the SMVD indication indicatesSMVD is used in the determination of the MV for the current codingblock: receive a first MV information associated with a first referencepicture list; and determine a second MV information associated with asecond reference picture list based on the first MV informationassociated with the first reference picture list, wherein a motionvector difference (MVD) associated with the first reference picture listand an MVD associated with the second reference picture list aresymmetric.
 22. The non-transitory computer-readable medium of claim 20,further comprising instructions for causing the one or more processorsto, based on a determination not to bypass BDOF for the current codingblock, refine an MV associated with a sub-block of the current codingblock based at least in part on gradients associated with a location inthe current coding block.